Zika vaccines and therapeutics: landscape analysis and challenges ahead

Zika vaccines and therapeutics: landscape analysis and challenges ahead Background: Various Zika virus (ZIKV) vaccine candidates are currently in development. Nevertheless, unique challenges in clinical development and regulatory pathways may hinder the licensure of high-quality, safe, and effective ZIKV vaccines. Discussion: Implementing phase 3 efficacy trials will be difficult given the challenges of the spatio-temporal heterogeneity of ZIKV transmission, the unpredictability of ZIKV epidemics, the broad spectrum of clinical manifestations making a single definite endpoint difficult, a lack of sensitive and specific diagnostic assays, and the need for inclusion of vulnerable target populations. In addition to a vaccine, drugs for primary prophylaxis, post-exposure prophylaxis, or treatment should also be developed to prevent or mitigate the severity of congenital Zika syndrome. Conclusion: Establishing the feasibility of immune correlates and/or surrogates are a priority. Given the challenges in conducting phase 3 trials at a time of waning incidence, human challenge trials should be considered to evaluate efficacy. Continued financial support and engagement of industry partners will be essential to the successful development, licensure, and accessibility of Zika vaccines or therapeutics. Keywords: Zika, Zika vaccines, Flavivirus, Anti-virals, Prophylaxis, Therapeutics, Efficacy trials, Zika diagnostics, Monoclonal antibodies, Immune correlates, Immune surrogates, Human controlled infections, Clinical endpoints Background having entered phase 2 trials (https://clinicaltrials.gov/ The devastating consequences of Zika virus (ZIKV) in- ct2/show/NCT03110770, https://clinicaltrials.gov/ct2/ fection, leading to congenital Zika syndrome (CZS) and show/NCT03014089). Additionally, the role of thera- neurological complications such as Guillain–Barre Syn- peutic and prophylactic medicinal products in the man- drome (GBS), led the World Health Organization agement of ZIKV infections in pregnant women and (WHO) to declare a Public Health Emergency of Inter- other high-risk groups remains to be determined. national Concern on February 1, 2016 [1], and to call on Herein, we describe the various vaccine platforms, with the global research and product development (R&D) a discussion on their advantages and disadvantages in communities to prioritize the development of preventa- the context of use scenarios, and provide an overview of tive and therapeutic solutions [2]. The R&D communi- the current status of vaccine development. Furthermore, ties responded rapidly, with 45 vaccine candidates being we propose three plausible clinical indications for initially evaluated in non-clinical studies and most pro- prophylactic or therapeutic agents against ZIKV. Both gressing to active development. Of these, several have vaccines and therapeutics must be evaluated for their advanced beyond pre-clinical studies in animals and en- efficacy in human trials, yet the design of efficacy trials tered phase 1 human trials [3, 4], with two candidates and the appropriate selection of clinical endpoints pose a challenge. In particular, the rapid decline in Zika cases in the second year following the Public Health * Correspondence: wildersmitha@who.int Emergency of International Concern declaration has put Immunization, Vaccines & Biologicals, World Health Organization, Geneva, Switzerland clinical efficacy trial feasibility at stake. We discuss op- Lee Kong Chian School of Medicine, Nanyang Technological University, tions on how best to address these hurdles. Singapore, Singapore Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 2 of 15 ZIKV vaccines and tick-borne encephalitis (TBEV), with well-defined WHO has outlined two use scenarios for a ZIKV vaccine correlates of protection, thus rendering the development [5], namely in emergency outbreak response and for en- of a monovalent vaccine against ZIKV with a favorable demic transmission. Emergency outbreak response involves probability of technical and regulatory success [8]. Early a targeted mass vaccination during an ongoing epidemic or findings from animal studies suggest a protective thresh- an imminent outbreak of ZIKV to prevent ZIKV-associated old of ZIKV vaccine-induced neutralizing activity that disease in women of child-bearing age in order to mitigate prevents viremia after acute infection, as determined CZS. Endemic transmission use involves a broad or univer- after challenge with an infective dose [13, 14]. Three dif- sal vaccination campaign of the general population in the ferent vaccine platforms have been tested in non-human inter-epidemic period, extending from early childhood to primate models, with all showing 100% protection adults, followed by routine immunization, in order to estab- against viremia following a ZIKV challenge [15, 16]. lish population immunity to prevent transmission, and ul- Additionally, various vaccine platforms have been tested timately to prevent ZIKV-related adverse birth outcomes for their ability to protect against ZIKV transmission to and neurological complications. the fetus [17], with the findings showing markedly di- Based on current knowledge on the transmission of minished levels of viral ZIKV RNA in maternal, placen- ZIKV and experiences with past disease outbreaks, tal, and fetal tissues, which resulted in protection against WHO has prioritized the development of vaccines suit- placental damage and fetal demise [17]. These studies able for use in an emergency or outbreak scenario. are therefore a proof-of-concept that protection against Therefore, and in line with the WHO Zika Strategic CZS is possible. Response plan, WHO developed a Target Product Profile for a ZIKV vaccine for emergency use where Potential hurdles to ZIKV vaccine development immunization of women of reproductive age is consid- Several important hurdles may impede ZIKV vaccine de- ered to be of highest priority [5]. Although WHO de- velopment. Firstly, given the early stages of development clared an end to its global health emergency over the of animal models for ZIKV infection, disease, maternal– spread of ZIKV on November 18, 2016, the long-term fetal transmission, and fetal infection, their relevance to need for a ZIKV vaccine continues [6]. Under the Blue- the human experience requires additional validation. print Plan of Action [7]. Current evidence suggests that even asymptomatic infec- WHO led a series of initiatives to maintain continuous tions with presumably low levels of viremia in the dialogue between developers, regulators, and public mother could result in CZS [18]. It is unknown whether health experts to identify how best to achieve rapid, ro- sterilizing immunity and robust T cell response are re- bust, safe, and evidence-based licensing of ZIKV vac- quired to avert transplacental transmission of ZIKV dur- cines. In June 2016, WHO hosted an expert consultation ing pregnancy [19]. Answering these questions will be on regulatory considerations for ZIKV vaccine develop- critical for the development of a vaccine that protects ment, outlining vaccine platform focal points for devel- against CZS. If sterilizing immunity is indeed required, opers and regulators, as well as the mechanisms of this would set a high bar for a ZIKV vaccine since, simi- approval [8]. In June 2017, additional information was lar to other flavivirus vaccines (e.g., JEV, dengue viruses provided regarding clinical trial endpoints and trial site (DENV), and TBEV), sterilizing immunity has not yet selection. WHO has also hosted periodic meetings to re- been achieved. Optimally, the efficacy afforded by a view the progress of ZIKV vaccine development and fos- ZIKV vaccine would be durable, as protection through- ter opportunities for data sharing [9, 10]. out the reproductive years is desired. Secondly, concerns have been raised about the hypo- Factors that render the development of a ZIKV vaccine thetical risk of vaccine-associated GBS given the associ- feasible ation of natural ZIKV infection with a higher risk of Although ZIKV strains are categorized into two genetic GBS [20, 21]. If the mechanism of ZIKV-associated GBS lineages, African and Asian/American, ZIKV has been is direct neuroinvasion, there could be implications for classified as a single serotype with limited strain variabil- the design of neurovirulence testing of live attenuated ity [11]. Recent studies on macaques showed that im- ZIKV vaccines [8]. Conversely, if GBS is immune medi- mune responses primed by infection with East African ated, there could be implications for all ZIKV vaccines. ZIKV completely protected macaques from detectable The sequence and antigenic similarity between ZIKV viremia when subsequently re-challenged with heterol- and DENV [22], and potentially also other flaviviruses, ogous Asian ZIKV [12]; thus, a ZIKV vaccine based on a has led some to speculate whether pre-existing immun- single ZIKV strain may be sufficient. Successful vaccines ity to one or more flaviviruses could impact clinical out- have been developed for other single serotype flavi- comes following a subsequent ZIKV infection, as many viruses such as yellow fever, Japanese encephalitis (JEV), of these flaviviruses co-circulate [23, 24]. Whilst in vitro Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 3 of 15 studies have generated evidence in support of immune transformation by insertional mutagenesis [31]. Con- enhancement [23] between DENV and ZIKV, an increas- versely, mRNA molecule-based vaccines act in the ing body of evidence from in vivo non-human primate cytoplasm and thus do not pose a risk of chromo- studies [25, 26] and observational studies in humans somal integration. [27] have shown a lack of association between more se- vere ZIKV disease and prior DENV infections, which is DNA ZIKV vaccines reassuring for vaccine development. Nevertheless, care- Inovio Pharmaceuticals and GeneOne Life Science, ful monitoring will be needed, and clinical trial study de- Inc. (KSE: 011000) have developed a synthetic, con- signs should ideally include evaluation of safety and sensus DNA vaccine (GLS-5700) encoding the ZIKV immunogenicity, as well as of the potential for clinical premembrane (prM) and envelope (E) proteins, ad- benefit in both flavivirus-primed and naive populations. ministered with the CELLECTRA -3P device, Inovio’s proprietary intradermal DNA delivery device. The Current ZIKV vaccine platforms delivery technology is based on electroporation. The Both traditional (purified inactivated, live attenuated, interim analysis of the phase 1, open-label clinical recombinant sub-unit) and more novel (DNA, self- trial at 14 weeks (i.e., after the third dose of vaccine replicating RNA, messenger RNA (mRNA), viral- givenina0–4 and 14 weeks schedule) evaluated the vectored) ZIKV vaccine platforms are in development. safety and immunogenicity of GLS-5700 in two In July 2016, WHO developed a catalog of preclinical groups of 20 participants each (NCT02809443) [32]. and clinical ZIKV vaccines by searching the WHO Inter- No serious adverse events were reported. After the national Clinical Trial Registry Platform [28] and the third vaccine dose, binding antibodies (as measured National Institutes of Health (NIH) clinical trial registry on enzyme-linked immunosorbent assay) were (ClinicalTrials.gov), by literature review, and by contact- detected in all participants. Neutralizing antibodies ing research groups in academia and industry. Table 1 developed in 62% of the vaccine recipients on the highlights the ZIKV vaccine candidates in clinical devel- Vero-cell assay. On a neuronal-cell assay, there was opment as of October 2017, and Table 2 outlines ZIKV 90% inhibition of ZIKV infection in the serum sam- vaccine candidates in the preclinical phase as of January ples of 70% of vaccine recipients and 50% inhibition 2017. Additionally, WHO maintains an updated list of in 95% of vaccine recipients. Further, the intraperito- ZIKV vaccine clinical trials through the WHO clinical tri- neal injection of post-vaccination serum protected als tracker [29]. Below, we discuss the potential advantages 103 of 112 (92%) IFNAR knockout mice that were and disadvantages of the various platforms, and highlight challenged with a lethal dose of ZIKV-PR209 strain. selected vaccines that have entered clinical trials. The US NIH Vaccine Research Center is advancing a ZIKV DNA vaccine candidate based on the technol- Nucleic acid vaccines ogy it developed for a highly immunogenic West Nile Nucleic acid vaccines have advanced the furthest in virus DNA vaccine [33], whereby the full coding se- clinical development. Both DNA plasmid-based vac- quences of the prM and E genes of ZIKV are inserted cines and mRNA vaccines have utility due to their into their DNA construct. In this manner, virus-like ease of production since encoding genes can easily be subviral particles are released after expression of prM replaced [30], and thus have potential for scalability and E [13]. The National Institute of Allergy and In- during an outbreak. They exhibit characteristics of fectious Diseases (NIAID) is using a needleless subunit vaccines and live attenuated vectors, with pressure-based delivery system developed by the com- conceptual safety advantages [22]. However, to date, pany PharmaJet, with results from immunogenicity neither a DNA nor an mRNA vaccine candidate has and protective efficacy studies in mice and in rhesus been evaluated in a phase 3 trial nor licensed for use monkeys indicating high levels of protection [13]. The in the prevention of another flavivirus infection, un- phase 1 clinical trial of this DNA vaccine started in like live, vectored, and inactivated vaccine platforms. September 2016 and a phase 2a clinical trial in Texas A limitation of DNA plasmid vaccines is the delivery and Puerto Rico was initiated in April 2017 [34]. A technology needed for optimal protein production. phase 2b trial is scheduled to begin before the end of For example, electroporation, i.e., the use of a pulsed 2017 in multiple sites with the potential for ZIKV electric field to introduce the DNA sequence into transmission [35]. cells [30], would make large scale deployment in low- resource settings more difficult. A potential concern mRNA vaccines with DNA vaccines is that there might be a small Modified ZIKV prM-E mRNA molecules were encapsu- possibility of chromosomal integration by non- lated in lipid nanoparticles in vaccine formulations [36, 37], homologous recombination, which may lead to cell showing complete protection in animal studies against Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 4 of 15 Table 1 WHO Zika virus vaccine pipeline: in human trials (last updated September 2017 [29]) Platform Candidate Immunogen Adjuvant Replicating Registry ID Trial status Sponsor name Sponsor type Phase Study start Age Sample size Location vaccine type virus date DNA GLS-5700 prM/E None No NCT02809443 Open, not GeneOne Life Industry Phase 1 1/7/16 Adult 40 United States recruiting Science, of America, Canada Inc./Inovio Pharmaceuticals NCT02887482 Open, GeneOne Life Industry Phase 1 1/8/16 Adult 160 Puerto Rico recruiting Science, Inc./Inovio Pharmaceuticals Peptide AGS-v Mosquito No NCT03055000 Open, NIH Government Phase 1 9/2/17 Adult 60 United States salivary recruiting of America proteins Recombinant MV-Zika prM/E None Yes NCT02996890 Open, Themis Industry Phase 1 4/4/17 Adult 48 Austria viral vector recruiting Bioscience mRNA mRNA-1325 prM/E None No NCT03014089 Open, Moderna Industry Phase 2 1/12/16 Adult 90 United States recruiting Therapeutics of America DNA VRC-ZKADNA085– prM/E None No NCT02840487 Open, not NIAID Government Phase 1 11/7/16 Adult 120 United States 00-VP or recruiting of America VRC-ZKADNA090–00-VP NCT02996461 Open, NIAID Government Phase 1 8/12/16 Adult 50 United States recruiting of America NCT03110770 Open, NIAID Government Phase 2 29/3/17 Child, Adult 2500 United States recruiting of America, Puerto Rico Inactivated ZIKV PIV Full genome Aluminum No NCT02963909 Open, NIAID Government Phase 1 1/11/16 Adult 75 United States whole target recruiting of America organism NCT02952833 Open, NIAID Government Phase 1 14/10/16 Adult 90 United States recruiting of America NCT02937233 Open, BIDMC Academic Phase 1 1/10/16 Adult 48 United States recruiting of America NCT03008122 Open, NIAID Government Phase 1 24/2/17 Adult 90 Puerto Rico recruiting Inactivated BBV121 Full genome Aluminum No CTRI/2017/05/ Open, Bharat Biotech Industry Phase 1 1/6/17 Adult 48 India whole target 008539 recruiting International organism Ltd., India BIDMC Beth Israel Deaconess Medical Center, NIAID National Institute of Allergy and Infectious Diseases, NIH National Institutes of Health, PIV purified, inactivated whole virus vaccines, ZIKV Zika virus Reference: http://www.who.int/immunization/research/vaccine_pipeline_tracker_spreadsheet/en/ Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 5 of 15 Table 2 WHO – Pipeline Zika virus (ZIKV) vaccines (in preclinical development) (last updated January 2017) January 2017) Platform Candidate Developer/Collaborators Replicating virus Antigen Adjuvant vaccine name (Yes/No) Inactivated whole BK1603 BIKEN No ZIKV full genome To be determined target organism Inactivated whole Bio-Manguinhos in house No ZIKV full genome Alum target organism development and Sanofi Pasteur/WRAIR (discontinued in 2017) Recombinant Bio-Manguinhos No ZIKV E protein Alum subunit (non- VLP) partnership Recombinant Bio-Manguinhos/Aggeu Yes PrM/E and PrM/E/NS1 None viral vector Magalhaes (FIOCRUZ) Live, attenuated Brazilian Ministry of Health Yes rZIKV None recombinant virus agreement with University of Texas rZIKV NS1 Inactivated whole Butantan ZIKV Butantan No ZIKV full genome Alum target organism Live, attenuated Butantan Butantan ZIKV full genome None target organism attenuated ZIKV Inactivated virus + ZIKV Emergent BioSolutions No ZIKV full genome Aluminum aluminum adjuvant Recombinant GEO-ZM05 GeoVax/University of No ZIKV PrM/E + NS1 None viral vector Georgia/CDC Atlanta, US SAM WT GSK-NIH Yes ZIKV prM/E SAM CO GSK-NIH Yes ZIKV prM/E SAM VRC_5283 GSK-NIH Yes ZIKV prM/E SAM VRC_5288 GSK-NIH Yes ZIKV prM/E Recombinant ZIK-80E Hawaii Biotech, Inc. No subunit (non-VLP) Recombinant ZIKVLP Institut Pasteur No subunit VLP Shanghai, China (non-fusion) Recombinant NI.LV-ZIK Institut Pasteur, Paris, No ZIKV prM/E None viral vector France Recombinant ChAdOx1-Zk Jenner Institute ZIKV prM/E None viral vector Inactivated whole KAKETSUKEN ZIKV Kaketsuken No ZIKV full genome TBD target organism Inactivated whole NewLink Genetics No ZIKV prM/E + NS1 TBD target organism Recombinant NewLink Genetics No ZIKV prM-E TBD subunit VLP (fusion) Live, attenuated rZIKV/D2D30 NIAID Yes ZIKV prM/E None target organism Live, attenuated rZIKV/D4D30 NIAID Yes ZIKV prM/E None target organism Live, attenuated rZIKVD30 NIAID Yes ZIKV full genome None target organism Recombinant NIAID Yes None viral vector Sf9 cells/Baculo ZIKV envelope Novavax, Inc. No ZIKV E protein Matrix M adjuvant or dimers (EnvD) aluminum hydroxide Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 6 of 15 Table 2 WHO – Pipeline Zika virus (ZIKV) vaccines (in preclinical development) (last updated January 2017) January 2017) (Continued) Platform Candidate Developer/Collaborators Replicating virus Antigen Adjuvant vaccine name (Yes/No) nanoDNA/ZIKA-LAMP LAMP-ZIKA Pharos Biologicals No ZIKV prM/E chimera construct nanoDNA Recombinant ZIKA recombinant Protein Sciences/Sinergium No ZIKV E protein Aluminum based subunit (non-VLP) Biotech/Lab Liomont Peptide Replikins Zika Replikins Ltd. No Synthetic Peptides None Vaccine and Bocker Recombinant Chimerivax-Zika Sanofi Pasteur ZIKV prM/E + NS1 None viral vector Recombinant SCV-CHIKV+ZIKV Sementis Ltd. No ZIKV, yellow fever, and None viral vector +YF CHIKV surface antigens Recombinant SCV-CHIKV+ZIKV Sementis Ltd. No ZIKV and CHIKV surface None viral vector antigens Recombinant SCV-ZIKV Sementis Ltd. No ZIKV surface antigens None viral vector Inactivated whole Takeda No ZIKV full genome Alum target organism Subunit Tours University No VLP HBV-Zika Prime-boost U1187 INSERM (CYROI, La Reunion) Live attenuated + ZIKV exosome Inactivated whole Valneva No ZIKV full genome Aluminum hydroxide target organism Recombinant viral vector VXA-Zikavax Vaxart ZIKV prM/E VLP VBI-2501A VBI Vaccines No ZIKV E + NS1 DNA WRAIR/BIDMC/Harvard No ZIKV prM/E Recombinant viral vector WRAIR/BIDMC/Harvard ZIKV prM/E BIDMC Beth Israel Deaconess Medical Center, BIKEN Research Foundation for Microbial Diseases of Osaka University, CDC Centers for Disease Control and Prevention, CHIKV Chikungunya virus, GSK GlaxoSmithKline, LAMP loop-mediated isothermal amplification, NIH National Institutes of Health, NIAID National Institute of Allergy and Infectious Diseases, SAM self-amplifying mRNA, SCV Sementis Copenhagen vector, VLP virus-like particles, WRAIR Walter Reed Army Institute of Research, YF Yellow fever, ZIKV Zika virus challenge after a single intradermal immunization [38]or and boosters to sustain protective immunity. ZIKV PIV de- after prime and boost intramuscular immunization [39]. rived from the Puerto Rico strain PRV ABC59 or from The nucleoside-modified mRNA ZIKV vaccine (mRNA- the MR 766 strain, produced in Vero cells, and inacti- 1325), which is being developed by Moderna, a Cambridge- vated with formalin, were tested in either Balb/c mice, based Biotech Company [36], entered a phase 1 clinical trial rhesus monkeys, AG 129 mice, or New Zealand white in December 2016 (NCT03014089). The mRNA candidate rabbits and showed good induction of ZIKV-specific developed by NIAID and GlaxoSmithKline could enter neutralizing antibodies [15, 16, 40]. Further, a ZIKV clinical trials in late 2017. PIV candidate with an alum adjuvant is being evaluated in several phase 1 trials (NCT03008122, NCT02952833, Purified, inactivated whole virus vaccines (PIV) NCT02963909, NCT02937233). The results of three The inactivation process eliminates virus replication phase 1 placebo-controlled, double-blind trials in while maintaining the antigenicity of the structural healthy adults of ZIKV PIV with aluminum hydroxide proteins, and thus PIV are thought to be safe during adjuvant were recently published [41], showing only pregnancy. PIV vaccines have been successfully licensed mild to moderate adverse events. By day 57, 92% of vac- for both JEV and TBEV. ZIKV PIV vaccines would most cine recipients had seroconverted (microneutralization likely be less costly than nucleic acid vaccines. However, titer ≥ 1:10), with peak geometric mean titres seen at day it is plausible that PIVs could require multiple doses in 43 and exceeding protective thresholds seen in animal the primary schedule, adjuvants to enhance immunogenicity, studies. NIAID’s Vaccine Research Center will test a ZIKV Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 7 of 15 PIV as a boost to its DNA Zika vaccine candidate. Bharat replicating virus. Subunit protein and VLP ZIKV vac- and Takeda are also developing a PIV against ZIKV. cines have not yet entered clinical evaluation. Live attenuated vaccines including recombinant Viral-vectored vaccine candidates heterologous flavivirus-vectored vaccines Viral-vectored vaccines share the same ease of pro- Live attenuated vaccines are usually a favored vaccine duction and stability with DNA plasmid vaccines and technology because of their ability to induce durable may therefore be easily scalable in epidemic situa- and effective adaptive immunity at relatively low tions. Viral-vectored vaccines induce both innate and production costs. Live vaccines mimic natural viral adaptive immune responses in mammalian hosts [42]. infections and thus induce a strong antibody and cell- Adenoviral vectors have been used to deliver ZIKV mediated immunity. However, live attenuated vaccines prM-E [40], and were shown to have higher neutral- induce transient low-grade viremia. As CZS is izing antibody titers and T-cell immunity than PIV, thought to occur even in asymptomatically infected DNA, and protein subunit vaccines [15]. Neverthe- pregnant women with low grade viremia [27], repli- less, limitations for adenovirus vaccines include their cating live vaccines need to be carefully evaluated for ability to induce toxic inflammatory responses and their safety prior to their administration to women of the potential for pre-existing immunity to naturally reproductive age, some of whom may be inadvertently occurring human adenoviruses resulting in acceler- pregnant. However, similar to the approach to con- ated clearance and dampened immunogenicity [42]. genital rubella syndrome [44, 45], live attenuated Zika Reactogenicity has been circumvented by the deletion vaccines may play a significant role in endemic trans- of genes required for replication, which also allows mission use, for example, by their incorporation to forlargerinserts [42]. Non-human primate adenovi- childhood vaccination programs in countries with ruses as vaccine vectors can bypass pre-existing im- ZIKV transmission. As ZIKV is a neurotropic virus, munity to human adenoviruses. Adenovirus-vectored neurovirulence and reproductive toxicology testing are and chimpanzee adenovirus-vectored vaccines for critical early steps in the development of live attenu- ZIKV are still in pre-clinical development. ated vaccines prior to human studies. Demonstration The core technology of the measles vector plat- of mosquito non-competence is also required. form developed at the Institut Pasteur in Paris and Live attenuated replication-competent vaccines are now licensed to Themis Bioscience was successfully available for recombinant (or chimeric) flaviviruses. tested in a phase 1 trial for chikungunya virus [43]. The principle of chimerization is to insert target an- The live recombinant measles virus-based chikun- tigens (for example, prM and E) into a back-bone gunya vaccine had good immunogenicity, even in the vector. Sanofi-Pasteur developed a recombinant ZIKV presence of anti-vector immunity, was safe, and had vaccine based on the yellow fever virus 17D back- a generally acceptable tolerability profile, making this bone, which has been used to develop and license the first promising measles virus-based candidate live attenuated recombinant DENV and JEV vaccines vaccine for use in humans. With regards to ZIKV, [46]. NIH/NIAID is also using recombinant DNA the measles vaccine-ZIKV chimeric virus recently en- technology to design recombinant ZIKV/DENV vi- tered a phase 1 clinical trial (NCT02996890). ruses, a strategy employed in the creation of the DENV-2 component of TV003, rDEN2/4Δ30 [47]. Subunit protein/virus-like particles (VLPs) For the ZIKV candidate vaccine, the prM and E cod- Subunit protein vaccines are attractive as a platform ing sequences of ZIKV are being evaluated, replacing due to their potential for safe use in all populations, those of DENV-2 or DENV-4. Combining the NIH including pregnant women, depending on adjuvants. tetravalent DENV vaccine with the recombinant Subunit protein vaccines are produced by transfect- ZIKV/DENV component may provide a combination ing a plasmid encoding a gene sequence of interest DENV-ZIKV vaccine, which could be useful for pop- into bacteria, yeast, or insect cells and utilizing the ulations living in regions endemic for both. machinery within those cells to produce the protein from the gene sequence. Similar to the PIV approach, WHO’s target product profile for a ZIKV vaccine a disadvantage to subunit protein vaccines is that Non-replicating platforms with no documented safety they are generally less immunogenic than live concerns for use during pregnancy would be the pre- vaccines and therefore require multiple doses and ferred vaccine platform for a ZIKV vaccine for emer- adjuvants to achieve protective immunity. The advan- gency use where women of reproductive age are the tage of VLPs is that the antigens are presented in primary target, ideally with a single dose primary their native conformation without the need for a series [6]. Vaccines based on replication-competent Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 8 of 15 platforms are likely to have profiles more suitable for Human immune globulin and anti-ZIKV monoclonal routine/endemic transmission use. As there is a the- antibodies (mAb) for prophylaxis or treatment oretical risk that live, attenuated, or replication- Human immune globulins are used clinically against competent viral vaccines given to pregnant women some viral infections in pregnant women. For measles, may be capable of crossing the placenta and infecting the primary purpose is to attenuate disease in the preg- the fetus [48], live vaccines are generally not recom- nant woman and prevent perinatal transmission to the mended for use during pregnancy. However, live at- newborn. For varicella, the purpose is to prevent or tenuated vaccines have been given to women of attenuate disease in the pregnant woman and prevent child-bearing age (MMR, yellow fever, polio) in situa- congenital infection [50]. However, the incubation time tions of increased risk of exposure, and inadvertent of varicella is 2–3 weeks, far longer than for ZIKV (3– vaccination of pregnant women does occur in mass 10 days), and therefore the critical time to treat is vaccination campaigns. To date, there is no evidence shorter for ZIKV. Plausibly, human immune globulin (or of increased adverse pregnancy outcomes due to hyperimmune globulin) from ZIKV-immune donors, or immunization with a live attenuated vaccine [49]. human mAbs, could be used for prophylaxis or therapy. However, the safety assessment and regulatory re- mAbs are promising because they can be precisely de- quirements for live attenuated/replicating-competent fined and their production controlled and scaled up. ZIKV vaccines are likely to require additional data Blood from a ZIKV-immune donor and a human B-cell compared to non-replicating vaccine platforms. Non- immortalization technique was used to identify human replicating vaccine platforms that either do not use mAbs that bound ZIKV antigens (NS1 and E proteins) any adjuvant or use a well-characterized adjuvant in [51]. An EDIII-specific antibody, ZKA190, protected currently licensed vaccines, such as aluminum salts mice from lethal ZIKV infection, illustrating the poten- (e.g., alum), would be preferable. However, the use of tial for antibody-based therapy. Another mAb, ZIKV- other adjuvants may be justifiable if accompanied 117, was identified as broadly neutralizing of ZIKV with superior performance and delivery aspects (e.g., infection in vitro [52]. Epitope mapping studies have re- reduced number of doses). vealed that ZIKV-117 recognized a unique quaternary epitope on the E protein dimer-dimer interface. Treat- Zika therapeutics ment of Zika-infected pregnant and non-pregnant mice Therapeutics against ZIKV need to be developed in with ZIKV-117 markedly reduced tissue pathology, pla- parallel to vaccines and may have a specific role in cental and fetal infection, and mortality. A bispecific reducing the burden of Zika infection and disease in mAb has also been developed that could address con- the populations most at risk of serious outcomes. cerns about the emergence of anti-viral resistance to Drugs could rationally be used for prophylaxis or monospecific mAbs [53]. Collectively, these data demon- post-exposure prophylaxis to prevent or mitigate the strate the feasibility of developing mAbs as therapeutic severity of CZS, and may have particular value when and/or prophylactic candidates. low endemicity does not justify widespread immunization. Aborting ongoing ZIKV shedding in Small molecule antivirals for prophylaxis or treatment seminal fluids may be another indication. Antivirals Multiple studies have demonstrated the anti-ZIKV activ- are the cornerstone of management of chronic hu- ity of several Food and Drug Administration (FDA)-ap- man viral infections like HIV, hepatitis B, and hepa- proved drugs or drug candidates being clinically tested titis C. There are also precedents for therapies to for other indications [54–59]. For example, the anti- manage viral infection in pregnant women and their HCV prodrug Sofosbuvir has anti-Zika virus activity in fetussuch aspost-exposure prophylaxiswith immune vitro [54]; however, repurposing this compound is prob- immunoglobulins in susceptible women to protect lematic because its hydrolysis is highly specific to the the mother and fetus from infection with varicella. liver. Niclosamide, a category B anthelmintic drug, Anynew drugsfor ZIKV wouldbeusedasanad- inhibited ZIKV replication at low micromolar concentra- junct to the standard of care for non-pregnant and tions [58]. However, the poor systemic bioavailability of pregnant persons, and may be indicated before vac- niclosamide is a hurdle to further clinical development cines become widely available or in addition to vac- against Zika. More than 20 out of 774 FDA-approved cine programs. drugs decreased ZIKV infection in an in vitro screening Three plausible clinical indications for application of a assay [54]. Selected compounds were further validated for medicinal prophylactic/therapeutic against ZIKV are (1) inhibition of ZIKV infection in human cervical, placental, to offer prophylaxis or early post-exposure prophylaxis, and neural stem cell lines, as well as in primary human (2) to accelerate viral clearance, and (3) to reduce amnion cells. Established anti-flaviviral drugs (e.g., borte- disease severity (Box 1). zomib and mycophenolic acid) and others with no Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 9 of 15 Box 1: Clinical indications for application of a prophylactic/therapeutic against Zika INDICATION 1. PROPHYLAXIS OR EARLY POST-EXPOSURE PROPHYLAXIS. (1) To prevent maternal infection and fetal disease: The objective is to prevent, or diminish, Zika virus (ZIKV) infection and disease in pregnant women, or women trying to become pregnant, and thus eliminate or substantially reduce the probability of intrauterine infection or transmission in the perinatal or postnatal period. Examples of this approach for Zika. Prophylaxis for a pregnant woman living in a setting where there is epidemic Zika transmission, or a pregnant traveler spending time in a Zika-affected location. Post-exposure prophylaxis might be considered where there is strong suspicion that a pregnant woman has been exposed to Zika because of epidemiological circumstances (e.g., lives in an endemic area and resides in a household where recent Zika cases have been diagnosed) Challenges. The risk/benefit of using prophylactic agents (small molecule drugs or immune globulin) must be balanced against the probability of the mother and fetus being infected and of that infection harming the fetus Examples of this approach for other infectious diseases. Varicella virus (chickenpox) infection during pregnancy may result in congenital varicella, which is usually benign and self-limiting, but can occasionally produce a characteristically severe pattern of abnormalities known as ‘congenital varicella syndrome’. Zoster immunoglobulin (a preparation of human Ig containing anti-varicella antibodies) is indicated for all pregnant women who have significant exposure to Varicella Zoster virus infection (defined as ‘living in the same household as a person with active chickenpox or herpes zoster or face-to-face contact with a person with chickenpox or uncovered zoster for at least 5 minutes’), who have no history of chickenpox and who are seronegative. (2) To prevent Guillain–Barre Syndrome or other ZIKV infection-related neurological complications. Prophylaxis throughout the duration of exposure (e.g., travel to a Zika endemic area). Early post-exposure prophylaxis after known exposure to a Zika case (e.g., sexual exposure, nosocomial exposure such as needle stick injury, living in same household of a current Zika case). INDICATION 2. THERAPY TO ACCELERATE RESOLUTION OF INFECTION Examples of this approach for Zika. A pregnant woman, who lives in a setting where ZIKV is known to circulate, presents to a clinic with clinical signs and symptoms that could represent ZIKV infection. Out of an abundance of caution, empiric treatment commences before the results of laboratory tests are known (if such tests are available). The purpose of treatment is to accelerate clearance of virus infection from the maternal tissues and mitigate the likelihood of intrauterine or peri- or postnatal virus infection. A second example of treatment is for neonates who have acquired ZIKV from intrauterine exposure or perinatally. Challenges. As for prophylaxis, the use of a therapeutic agent must be balanced by the safety and cost profile of the drug and the likelihood that treatment will deliver clinical benefits to the fetus, i.e., prevent or modify intrauterine infection. For many Zika cases, viremia is already in rapid decline, or even undetectable by the time the patient presents to healthcare providers. An additional delay is created if treatment is guided by laboratory diagnostics rather than an empiric approach. Examples of this approach for other infectious diseases. There are no examples for acute viral infections, but in the setting of chronic infections such as HIV, it is well accepted that vertical transmission is RNA copy number-dependent, with higher rates occurring with increasing viral loads present in the mother. Treatment of pregnant women has been demonstrated to drastically reduce the incidence of vertical transmission in women undergoing treatment with a combination of antiretroviral compounds [82]. INDICATION 3. DISEASE MODIFICATION FOR EXISTING CONGENITAL INFECTION. The objective is the treatment of existing intrauterine fetal infection by eradication of virus and thus reduce the severity of congenital Zika syndrome. Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 10 of 15 Examples of this approach for Zika. A pregnant woman has amniocentesis performed because of concerns about recent exposure to ZIKV. The amniocentesis fluid is RT-PCR positive for ZIKV. Treatment is commenced to eradicate virus from the fetal tissues. Challenges. This indication is unlikely to be the sole basis for drug development. Clinical trials to test for improvements in fetal outcome would be very long in duration and likely prohibitively expensive to perform. The extent of viremia in the pregnant woman before commencement of treatment may already have led to significant pathology of the fetus. Examples of this approach for other infectious diseases. Unfortunately, there is no evidence that the outcome of an established congenital viral infection (e.g., with cytomegalovirus or varicella) can me modified by small molecule drug or immunoglobulin treatment. previously known antiviral activity (e.g., daptomycin) were the same model (Cristina Cassetti; personal communica- identified as inhibitors of ZIKV infection. These results tion). A summary of compounds found to have Zika anti- offer the possibility of a repurposed drug being used for viral properties in vitro (Table 3) and of some of the Zika therapeutic or prophylactic indications. repurposed drugs reported to have anti-Zika activities are Newly discovered candidate anti-virals include a synthetic provided herein (Table 4). peptide derived from the stem region of the ZIKV envelope protein, designated Z2, which potently inhibits infection of Challenges for clinical evaluation of Zika vaccines ZIKV and other flaviviruses in vitro [60]. Z2 is able to pene- and therapeutics trate the placental barrier to enter fetal tissues and prevent Various challenges may delay or hinder the successful licen- vertical transmission of ZIKV in pregnant C57BL/6 mice sure of Zika vaccines or therapeutics, as described below. [60]. Another molecule, galidesivir, is an adenosine analogue active in cell culture against a wide-range of RNA Selection of the most suitable clinical endpoint viruses [61]. Galidesivir treatment of ZIKV-infected mice In June 2017, WHO convened a meeting to elaborate on significantly improved survival even when treatment was clinical endpoints for ZIKV vaccine efficacy trials [10]. Al- initiated 5 days after infection [62]. However, potential hur- though preventing CZS is the outcome of greatest interest dles for galidesivir development is the requirement for an for public health, the large sample sizes required, the focus oral formulation (galidesivir requires parenteral administra- on women only, the heterogeneity of clinical manifesta- tion). Ribavirin, another broad-spectrum but teratogenic tions of CZS, and ethical considerations render CZS as antiviral, did not improve outcomes from ZIKV infection in the primary endpoint unfeasible. A possible endpoint for Table 3 List of potential compounds for repurposing with anti-Zika activity, extracted from [19, 83] Drug group Drug name Description Nucleoside analogs Sofosbuvir, MK-608 • Inhibit Zika virus (ZIKV) replication in cellular assays • Efficacious in animal models 2CMC, Ribavirin, • Showed antiviral activity in cell culture Favipiravir, T1105 BCX4430, GS5734 • Reduced mortality in ZIKV-infected mice • Currently in phase I and II clinical trials Peptidomimetic agents CN-716 • Inhibit ZIKV protease in vitro, but only weakly inhibit viral replication • Due to safety reasons, may not translate as therapeutic option for pregnant women, but could be applied to other infected individuals Adenosine analog NITD008 • Showed potent anti-ZIKV activity • Could serve as a reference inhibitor for future drug screen and discovery Cyclin-dependent PHA-690509 • Showed inhibition of ZIKV replication of all three strains kinase inhibitor Antimalaria Chloroquine • Reduces virus production, the number of infected cells, and cell death promoted by ZIKV infection without any cytotoxic effect • Promising candidate for ZIKV clinical trials • Can be safely administered to pregnant women since it is clinically approved Anthelmintic Bithionol • Propagate by activating host caspases and inducing programmed cell death Epigallocatechin gallate • Natural compound found in food items, particularly green tea • It inhibits ZIKV entry into host cell Interferon-inducible • Inhibit the replication of a number of pathogenic viruses transmembrane proteins Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 11 of 15 Table 4 High throughput screening for potential compounds with anti-Zika activity (drug repurposing) Study No. of drugs Compounds identified with anti-Zika activity Remarks screened Barrows et al. [54] 774 FDA-approved Clofazimine, Digoxin, Gemcitabine, Ivermectin, • Mycophenolic acid (MPA), Ivermectin, agents Mefloquine, Mercaptopurine hydrate, Mycophenolic Daptomycin, Mefloquine, Palonosetron acid, Fingolimod, Mycophenolate mofetil, Dactinomycin, identified as having higher potency Bortezomib, Methoxsalen (Xanthotoxin), Azathioprine, • Daptomycin, Mefloquine, and Palonosetron are Thioguanine, Auranofin, Sertraline, Pyrimethamine, pregnancy category B drugs Daptomycin, Palonosetron, Deferasirox, Micafungin, Sorafenib tosylate, Cyclosporine A, Mebendazole Xu et al. [58] 6000 compounds, Niclosamide, Emricasan, 10 structurally unrelated • Emricasan is an inhibitor of caspase-3 activity > 2000 inhibitors of CDK but uncertain if a capase-3 inhibitor with FDA-approved agents anti-inflammatory properties impacts development of unborn fetus clinical trials could be ZIKV infection (whether symptom- and ensuring that pregnant women have fair access to atic or not), which would require a smaller sample size participate in ZIKV vaccine trials that offer a favorable compared to a clinical endpoint. However, detecting ratio of risks to potential benefits. Clinical development asymptomatic ZIKV infections (as measured by serocon- plans should therefore include systematic collection of version or sampling for virological detection) poses several relevant indicators and outcomes of safety and efficacy challenges, including the requirement of very frequent for pregnant women. Although certainly a complex chal- blood, urine, and possibly semen collection so as not to lenge, a concerted and proactive effort is required to ad- miss the acute infection and achieve virological diagnosis dress the needs of pregnant women and their offspring [63]. Vaccination may also interfere with serological test- early and across the ZIKV vaccine R&D pathway. ing, e.g., it may render it difficult to discriminate between vaccine response and natural infection. A challenge with Sample size and trial site selections using clinical disease as the primary endpoint is that ZIKV Generating clinical efficacy data in a reasonable sample illness is often associated with mild and non-specific size and an acceptable timeframe and cost is challenging symptoms, which raises challenges for case detection. A at a time when global Zika incidence has declined to low standardized clinical case definition is essential to facilitate levels. Areas with recent active ZIKV transmission may the comparison and combining of information from dif- not be the best sites for clinical trials. Given that estimates ferent studies. A working case definition of virologically of ZIKV seroprevalence are as high as 70% in some areas confirmed Zika illness has been provided by the Pan that experienced an outbreak, the proportion of suscep- American Health Organization [64]. tible individuals in such populations will be low, with a The consensus at the WHO technical consultation in subsequent incidence too low to sustain an efficacy trial. June 2017 was to select virologically confirmed clinical ill- Therefore, the WHO technical consultation in June 2017 ness as the primary endpoint, and to additionally study a proposed the projection of future evolution of the ZIKV subset to explore the protection against infection or reduc- epidemic based on the presence and vectorial capacity of tion in viremia. The underlying assumption is that reduc- Aedes mosquitoes [66, 67], travel patterns [68–70], and tion in ZIKV disease incidence is associated with either risk mapping and modeling [71–74] to predict the move- sterilizing immunity or a reduction in ZIKV viremia, which ment of Zika [75, 76]; various mathematical modeling in turn will reduce or prevent subsequent development of groups are working to this end. A multi-site approach for complications in pregnant and non-pregnant individuals. vaccine trials will be needed to increase the chance of in- cluding populations with a high incidence of disease, as Inclusion of pregnant women in trial design and safety well as providing an opportunity to evaluate vaccine effi- considerations cacy across different populations. Although pregnant women would not be the primary target population for efficacy trials based upon the above rationale, pregnant women remain a priority population Immune correlates for ZIKV vaccine use in areas experiencing ongoing An immune correlate of protection is an immune transmission and in future outbreaks. Thus, the Ethics response marker that is statistically associated with pro- Working Group on ZIKV Research and Pregnancy [65] tection from disease or infection and may be either recommended the collection of data specific to safety mechanistic (causally related to outcome) or non- and immunogenicity in pregnancy for all ZIKV vaccine mechanistic/surrogate (statistically related to outcome). candidates to which pregnant women may be exposed Given the global decline in cases, it is unclear whether Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 12 of 15 large scale efficacy trials are viable given the current in- for at least 3–5years from thetimeofcompletionofpri- cidence of ZIKV transmission. If clinical efficacy trials mary vaccination due to the concern of immune enhance- are not feasible, immune correlates/surrogates derived ment [80]; however, given the lack of data supporting a from passive protection studies in animals, natural his- clinical significant interaction between DENV and ZIKV tory studies, and controlled human challenge study re- [26–28], such a formal recommendation has not yet been sults may possibly represent acceptable endpoint data made for Zika vaccine development. Nevertheless, a longer for initial emergency use authorization and eventual follow-up period to monitor safety could be considered. licensure. ‘Accelerated approval’ is based on the demon- stration of a surrogate of protection though well- Establishing a transparent framework for selecting vaccines controlled clinical studies that are reasonably likely to Given the global decline in ZIKV incidence and the po- predict clinical benefit. The US FDA ‘animal rule’ is tential bottleneck in identifying suitable trial sites, a pro- based on the demonstration of an immune marker of posal was made during the June 2017 WHO technical protection in animal models that will reasonably likely consultation to establish a transparent framework for predict clinical benefits in humans. Both accelerated ap- prioritizing vaccines to be evaluated in phase 2b/3 trials. proval and animal rule approaches require post- Selection criteria would depend on the desired attri- licensure studies to verify clinical benefit and safety. butes, including compliance with the target product pro- Controlled human infection models are a promising av- file, pre-clinical evidence of complete or near-complete enue to explore immune correlates in humans, however, prevention or reduction of viremia, safety during preg- they are associated with complex ethical considerations. nancy, and scalability of the product. The feasibility of establishing immune correlates or sur- rogates is now a priority. Donor and industry fatigue Major vaccine producers, government-funded insti- Assay optimization and standardization tutions, academics, and small to mid-size research A comprehensive review of ZIKV diagnostics was recently enterprises responded promptly to the Zika out- performed [77] and shortcomings highlighted [63]. In the break, setting aside other activities to focus on rap- context of a highly epidemic disease with an apparent short idly developing vaccines and therapeutics against duration of detectable viremia and relatively infrequent inci- Zika, supported by government and philanthropic dence of clinical disease, reliable case ascertainment in effi- funding agencies. However, with the rapid decline in cacy trials is critical. However, the short and relatively low cases, the unpredictability of future outbreaks, and level viremia is difficult to detect, and the serological assays the still poorly defined use scenarios, the commer- lack specificity because of cross-reactivity between other co- cial market has become questionable. The prospect circulating flaviviruses and flavivirus vaccines [78]. Frequent of a licensed Zika vaccine is at stake unless govern- sampling over time and sampling of various bodily fluids ments and other donors sustain the level of support (whole blood, serum, urine), as well as the combination of to advance development. Current models for stimu- various diagnostic assays will be necessary to increase the lating epidemic product development are failing. The diagnostic yield. For the comparability of clinical trial re- Coalition for Epidemic Preparedness Innovations sults, it is crucial to standardize diagnostic assays used (CEPI) is a new alliance between governments, in- and immunological reference reagents should be available. dustry, academia, philanthropy, intergovernmental The plaque reduction neutralization test is still considered institutions (such as WHO), and civil society, and to be the laboratory standard against which other neutral- was founded to finance and coordinate the develop- izing antibody assays should be compared. A guideline on ment of new vaccines to prevent and contain infec- plaque reduction neutralization test standardization can tious disease epidemics [81]. Zika is not yet on the be found on the specific WHO website [79]. priority list for CEPI, but as donor and industry fa- tigue may increase, CEPI, or such other mecha- Interaction between DENV and ZIKV nisms, will be needed to ensure that, out of the Given the widespread endemicity of DENV in the areas many Zika vaccine candidates, at least one will make most affected by the current ZIKV outbreak, and the fact it to the finish line. that short- or long-term immunological interaction be- tween DENV and ZIKV cannot currently be excluded, trials Conclusion would ideally need to take baseline blood samples for all At least 45 Zika vaccine candidates have been or are in subjects to ascertain prior DENV exposure in order to development, some of them already in phase 2 clinical study the impact of prior immunity to DENV on vaccine trials. Multiple vaccine platforms have shown robust performance and safety. For DENV vaccines, WHO recom- protection against ZIKV challenge in animal models. mends that subjects are followed-up for safety and efficacy However, unique challenges will need to be addressed in Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 13 of 15 the clinical development and regulatory pathways of a Ethics approval and consent to participate Not applicable. ZIKV vaccine that may hinder the development, licen- sure, and WHO-prequalification of high-quality, safe, Competing interests and effective ZIKV vaccines. Implementing phase 3 effi- JH is an employee of the World Health Organization (WHO), as was KV at the cacy trials will be difficult given the challenges of the time of this work. AWS is consultant to WHO. The authors alone are responsible for the views expressed in this publication and they do not spatial and temporal heterogeneity of ZIKV transmis- necessarily represent the decisions or policies of WHO. The authors declare sion, the unpredictability of the ZIKV epidemics, the no conflicts of interest. broad spectrum of clinical manifestations making a sin- gle definite endpoint difficult, the lack of sensitive and Publisher’sNote specific diagnostic assays, and the need for inclusion of Springer Nature remains neutral with regard to jurisdictional claims in vulnerable target populations. In addition to a vaccine, published maps and institutional affiliations. drugs for primary prophylaxis, post-exposure prophylaxis, Author details or treatment should also be developed in order to prevent 1 Immunization, Vaccines & Biologicals, World Health Organization, Geneva, or mitigate the severity of CZS. The global research and Switzerland. Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore, Singapore. Department of Epidemiology and Global public health community should prioritize the develop- Health, Umea University, Umea, Sweden. Center for Immunization Research, ment of ZIKV vaccines and therapeutics that will be ac- Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA. ceptable for use by women of reproductive age, and ensure State University of New York, Upstate Medical University, Syracuse, NY, USA. Doherty Institute for Infection and Immunity, Parkville, VIC 3010, Australia. availability and affordability for use in countries where 7 8 The Royal Melbourne Hospital, Parkville, VIC 3010, Australia. Oxford ZIKV is circulating. To this end, WHO is working towards University Clinical Research Unit, 764 Vo Van Kiet street, District 5, Ho Chi a roadmap for Zika vaccine and product development. Minh City, Vietnam. Institute of Vector-borne Disease, Monash University, Melbourne, VIC, Australia. Abbreviations Received: 30 October 2017 Accepted: 1 May 2018 CEPI: Coalition for Epidemic Preparedness Innovations; CZS: congenital Zika syndrome; DENV: Dengue virus; FDA: Food and Drug Administration; GBS: Guillain–Barre syndrome; JEV: Japanese encephalitis virus; mAb: monoclonal antibodies; mRNA: messenger RNA; NIH: National Institutes References of Health; PIV: purified, inactivated whole virus vaccines; R&D: research and 1. Heymann DL, Hodgson A, Sall AA, Freedman DO, Staples JE, Althabe F, development; TBEV: tick-borne encephalitis virus; VLP: virus-like particles; Baruah K, Mahmud G, Kandun N, Vasconcelos PF, et al. Zika virus and WHO: World Health Organization; ZIKV: Zika virus microcephaly: why is this situation a PHEIC? Lancet. 2016;387(10020):719–21. 2. World Health Organization. WHO and Experts Prioritize Vaccines, Diagnostics and Innovative Vector Control Tools for Zika R&D. http://www.who.int/mediacentre/ Acknowledgements news/notes/2016/research-development-zika/en. 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Zika vaccines and therapeutics: landscape analysis and challenges ahead

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Medicine & Public Health; Medicine/Public Health, general; Biomedicine, general
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Abstract

Background: Various Zika virus (ZIKV) vaccine candidates are currently in development. Nevertheless, unique challenges in clinical development and regulatory pathways may hinder the licensure of high-quality, safe, and effective ZIKV vaccines. Discussion: Implementing phase 3 efficacy trials will be difficult given the challenges of the spatio-temporal heterogeneity of ZIKV transmission, the unpredictability of ZIKV epidemics, the broad spectrum of clinical manifestations making a single definite endpoint difficult, a lack of sensitive and specific diagnostic assays, and the need for inclusion of vulnerable target populations. In addition to a vaccine, drugs for primary prophylaxis, post-exposure prophylaxis, or treatment should also be developed to prevent or mitigate the severity of congenital Zika syndrome. Conclusion: Establishing the feasibility of immune correlates and/or surrogates are a priority. Given the challenges in conducting phase 3 trials at a time of waning incidence, human challenge trials should be considered to evaluate efficacy. Continued financial support and engagement of industry partners will be essential to the successful development, licensure, and accessibility of Zika vaccines or therapeutics. Keywords: Zika, Zika vaccines, Flavivirus, Anti-virals, Prophylaxis, Therapeutics, Efficacy trials, Zika diagnostics, Monoclonal antibodies, Immune correlates, Immune surrogates, Human controlled infections, Clinical endpoints Background having entered phase 2 trials (https://clinicaltrials.gov/ The devastating consequences of Zika virus (ZIKV) in- ct2/show/NCT03110770, https://clinicaltrials.gov/ct2/ fection, leading to congenital Zika syndrome (CZS) and show/NCT03014089). Additionally, the role of thera- neurological complications such as Guillain–Barre Syn- peutic and prophylactic medicinal products in the man- drome (GBS), led the World Health Organization agement of ZIKV infections in pregnant women and (WHO) to declare a Public Health Emergency of Inter- other high-risk groups remains to be determined. national Concern on February 1, 2016 [1], and to call on Herein, we describe the various vaccine platforms, with the global research and product development (R&D) a discussion on their advantages and disadvantages in communities to prioritize the development of preventa- the context of use scenarios, and provide an overview of tive and therapeutic solutions [2]. The R&D communi- the current status of vaccine development. Furthermore, ties responded rapidly, with 45 vaccine candidates being we propose three plausible clinical indications for initially evaluated in non-clinical studies and most pro- prophylactic or therapeutic agents against ZIKV. Both gressing to active development. Of these, several have vaccines and therapeutics must be evaluated for their advanced beyond pre-clinical studies in animals and en- efficacy in human trials, yet the design of efficacy trials tered phase 1 human trials [3, 4], with two candidates and the appropriate selection of clinical endpoints pose a challenge. In particular, the rapid decline in Zika cases in the second year following the Public Health * Correspondence: wildersmitha@who.int Emergency of International Concern declaration has put Immunization, Vaccines & Biologicals, World Health Organization, Geneva, Switzerland clinical efficacy trial feasibility at stake. We discuss op- Lee Kong Chian School of Medicine, Nanyang Technological University, tions on how best to address these hurdles. Singapore, Singapore Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 2 of 15 ZIKV vaccines and tick-borne encephalitis (TBEV), with well-defined WHO has outlined two use scenarios for a ZIKV vaccine correlates of protection, thus rendering the development [5], namely in emergency outbreak response and for en- of a monovalent vaccine against ZIKV with a favorable demic transmission. Emergency outbreak response involves probability of technical and regulatory success [8]. Early a targeted mass vaccination during an ongoing epidemic or findings from animal studies suggest a protective thresh- an imminent outbreak of ZIKV to prevent ZIKV-associated old of ZIKV vaccine-induced neutralizing activity that disease in women of child-bearing age in order to mitigate prevents viremia after acute infection, as determined CZS. Endemic transmission use involves a broad or univer- after challenge with an infective dose [13, 14]. Three dif- sal vaccination campaign of the general population in the ferent vaccine platforms have been tested in non-human inter-epidemic period, extending from early childhood to primate models, with all showing 100% protection adults, followed by routine immunization, in order to estab- against viremia following a ZIKV challenge [15, 16]. lish population immunity to prevent transmission, and ul- Additionally, various vaccine platforms have been tested timately to prevent ZIKV-related adverse birth outcomes for their ability to protect against ZIKV transmission to and neurological complications. the fetus [17], with the findings showing markedly di- Based on current knowledge on the transmission of minished levels of viral ZIKV RNA in maternal, placen- ZIKV and experiences with past disease outbreaks, tal, and fetal tissues, which resulted in protection against WHO has prioritized the development of vaccines suit- placental damage and fetal demise [17]. These studies able for use in an emergency or outbreak scenario. are therefore a proof-of-concept that protection against Therefore, and in line with the WHO Zika Strategic CZS is possible. Response plan, WHO developed a Target Product Profile for a ZIKV vaccine for emergency use where Potential hurdles to ZIKV vaccine development immunization of women of reproductive age is consid- Several important hurdles may impede ZIKV vaccine de- ered to be of highest priority [5]. Although WHO de- velopment. Firstly, given the early stages of development clared an end to its global health emergency over the of animal models for ZIKV infection, disease, maternal– spread of ZIKV on November 18, 2016, the long-term fetal transmission, and fetal infection, their relevance to need for a ZIKV vaccine continues [6]. Under the Blue- the human experience requires additional validation. print Plan of Action [7]. Current evidence suggests that even asymptomatic infec- WHO led a series of initiatives to maintain continuous tions with presumably low levels of viremia in the dialogue between developers, regulators, and public mother could result in CZS [18]. It is unknown whether health experts to identify how best to achieve rapid, ro- sterilizing immunity and robust T cell response are re- bust, safe, and evidence-based licensing of ZIKV vac- quired to avert transplacental transmission of ZIKV dur- cines. In June 2016, WHO hosted an expert consultation ing pregnancy [19]. Answering these questions will be on regulatory considerations for ZIKV vaccine develop- critical for the development of a vaccine that protects ment, outlining vaccine platform focal points for devel- against CZS. If sterilizing immunity is indeed required, opers and regulators, as well as the mechanisms of this would set a high bar for a ZIKV vaccine since, simi- approval [8]. In June 2017, additional information was lar to other flavivirus vaccines (e.g., JEV, dengue viruses provided regarding clinical trial endpoints and trial site (DENV), and TBEV), sterilizing immunity has not yet selection. WHO has also hosted periodic meetings to re- been achieved. Optimally, the efficacy afforded by a view the progress of ZIKV vaccine development and fos- ZIKV vaccine would be durable, as protection through- ter opportunities for data sharing [9, 10]. out the reproductive years is desired. Secondly, concerns have been raised about the hypo- Factors that render the development of a ZIKV vaccine thetical risk of vaccine-associated GBS given the associ- feasible ation of natural ZIKV infection with a higher risk of Although ZIKV strains are categorized into two genetic GBS [20, 21]. If the mechanism of ZIKV-associated GBS lineages, African and Asian/American, ZIKV has been is direct neuroinvasion, there could be implications for classified as a single serotype with limited strain variabil- the design of neurovirulence testing of live attenuated ity [11]. Recent studies on macaques showed that im- ZIKV vaccines [8]. Conversely, if GBS is immune medi- mune responses primed by infection with East African ated, there could be implications for all ZIKV vaccines. ZIKV completely protected macaques from detectable The sequence and antigenic similarity between ZIKV viremia when subsequently re-challenged with heterol- and DENV [22], and potentially also other flaviviruses, ogous Asian ZIKV [12]; thus, a ZIKV vaccine based on a has led some to speculate whether pre-existing immun- single ZIKV strain may be sufficient. Successful vaccines ity to one or more flaviviruses could impact clinical out- have been developed for other single serotype flavi- comes following a subsequent ZIKV infection, as many viruses such as yellow fever, Japanese encephalitis (JEV), of these flaviviruses co-circulate [23, 24]. Whilst in vitro Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 3 of 15 studies have generated evidence in support of immune transformation by insertional mutagenesis [31]. Con- enhancement [23] between DENV and ZIKV, an increas- versely, mRNA molecule-based vaccines act in the ing body of evidence from in vivo non-human primate cytoplasm and thus do not pose a risk of chromo- studies [25, 26] and observational studies in humans somal integration. [27] have shown a lack of association between more se- vere ZIKV disease and prior DENV infections, which is DNA ZIKV vaccines reassuring for vaccine development. Nevertheless, care- Inovio Pharmaceuticals and GeneOne Life Science, ful monitoring will be needed, and clinical trial study de- Inc. (KSE: 011000) have developed a synthetic, con- signs should ideally include evaluation of safety and sensus DNA vaccine (GLS-5700) encoding the ZIKV immunogenicity, as well as of the potential for clinical premembrane (prM) and envelope (E) proteins, ad- benefit in both flavivirus-primed and naive populations. ministered with the CELLECTRA -3P device, Inovio’s proprietary intradermal DNA delivery device. The Current ZIKV vaccine platforms delivery technology is based on electroporation. The Both traditional (purified inactivated, live attenuated, interim analysis of the phase 1, open-label clinical recombinant sub-unit) and more novel (DNA, self- trial at 14 weeks (i.e., after the third dose of vaccine replicating RNA, messenger RNA (mRNA), viral- givenina0–4 and 14 weeks schedule) evaluated the vectored) ZIKV vaccine platforms are in development. safety and immunogenicity of GLS-5700 in two In July 2016, WHO developed a catalog of preclinical groups of 20 participants each (NCT02809443) [32]. and clinical ZIKV vaccines by searching the WHO Inter- No serious adverse events were reported. After the national Clinical Trial Registry Platform [28] and the third vaccine dose, binding antibodies (as measured National Institutes of Health (NIH) clinical trial registry on enzyme-linked immunosorbent assay) were (ClinicalTrials.gov), by literature review, and by contact- detected in all participants. Neutralizing antibodies ing research groups in academia and industry. Table 1 developed in 62% of the vaccine recipients on the highlights the ZIKV vaccine candidates in clinical devel- Vero-cell assay. On a neuronal-cell assay, there was opment as of October 2017, and Table 2 outlines ZIKV 90% inhibition of ZIKV infection in the serum sam- vaccine candidates in the preclinical phase as of January ples of 70% of vaccine recipients and 50% inhibition 2017. Additionally, WHO maintains an updated list of in 95% of vaccine recipients. Further, the intraperito- ZIKV vaccine clinical trials through the WHO clinical tri- neal injection of post-vaccination serum protected als tracker [29]. Below, we discuss the potential advantages 103 of 112 (92%) IFNAR knockout mice that were and disadvantages of the various platforms, and highlight challenged with a lethal dose of ZIKV-PR209 strain. selected vaccines that have entered clinical trials. The US NIH Vaccine Research Center is advancing a ZIKV DNA vaccine candidate based on the technol- Nucleic acid vaccines ogy it developed for a highly immunogenic West Nile Nucleic acid vaccines have advanced the furthest in virus DNA vaccine [33], whereby the full coding se- clinical development. Both DNA plasmid-based vac- quences of the prM and E genes of ZIKV are inserted cines and mRNA vaccines have utility due to their into their DNA construct. In this manner, virus-like ease of production since encoding genes can easily be subviral particles are released after expression of prM replaced [30], and thus have potential for scalability and E [13]. The National Institute of Allergy and In- during an outbreak. They exhibit characteristics of fectious Diseases (NIAID) is using a needleless subunit vaccines and live attenuated vectors, with pressure-based delivery system developed by the com- conceptual safety advantages [22]. However, to date, pany PharmaJet, with results from immunogenicity neither a DNA nor an mRNA vaccine candidate has and protective efficacy studies in mice and in rhesus been evaluated in a phase 3 trial nor licensed for use monkeys indicating high levels of protection [13]. The in the prevention of another flavivirus infection, un- phase 1 clinical trial of this DNA vaccine started in like live, vectored, and inactivated vaccine platforms. September 2016 and a phase 2a clinical trial in Texas A limitation of DNA plasmid vaccines is the delivery and Puerto Rico was initiated in April 2017 [34]. A technology needed for optimal protein production. phase 2b trial is scheduled to begin before the end of For example, electroporation, i.e., the use of a pulsed 2017 in multiple sites with the potential for ZIKV electric field to introduce the DNA sequence into transmission [35]. cells [30], would make large scale deployment in low- resource settings more difficult. A potential concern mRNA vaccines with DNA vaccines is that there might be a small Modified ZIKV prM-E mRNA molecules were encapsu- possibility of chromosomal integration by non- lated in lipid nanoparticles in vaccine formulations [36, 37], homologous recombination, which may lead to cell showing complete protection in animal studies against Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 4 of 15 Table 1 WHO Zika virus vaccine pipeline: in human trials (last updated September 2017 [29]) Platform Candidate Immunogen Adjuvant Replicating Registry ID Trial status Sponsor name Sponsor type Phase Study start Age Sample size Location vaccine type virus date DNA GLS-5700 prM/E None No NCT02809443 Open, not GeneOne Life Industry Phase 1 1/7/16 Adult 40 United States recruiting Science, of America, Canada Inc./Inovio Pharmaceuticals NCT02887482 Open, GeneOne Life Industry Phase 1 1/8/16 Adult 160 Puerto Rico recruiting Science, Inc./Inovio Pharmaceuticals Peptide AGS-v Mosquito No NCT03055000 Open, NIH Government Phase 1 9/2/17 Adult 60 United States salivary recruiting of America proteins Recombinant MV-Zika prM/E None Yes NCT02996890 Open, Themis Industry Phase 1 4/4/17 Adult 48 Austria viral vector recruiting Bioscience mRNA mRNA-1325 prM/E None No NCT03014089 Open, Moderna Industry Phase 2 1/12/16 Adult 90 United States recruiting Therapeutics of America DNA VRC-ZKADNA085– prM/E None No NCT02840487 Open, not NIAID Government Phase 1 11/7/16 Adult 120 United States 00-VP or recruiting of America VRC-ZKADNA090–00-VP NCT02996461 Open, NIAID Government Phase 1 8/12/16 Adult 50 United States recruiting of America NCT03110770 Open, NIAID Government Phase 2 29/3/17 Child, Adult 2500 United States recruiting of America, Puerto Rico Inactivated ZIKV PIV Full genome Aluminum No NCT02963909 Open, NIAID Government Phase 1 1/11/16 Adult 75 United States whole target recruiting of America organism NCT02952833 Open, NIAID Government Phase 1 14/10/16 Adult 90 United States recruiting of America NCT02937233 Open, BIDMC Academic Phase 1 1/10/16 Adult 48 United States recruiting of America NCT03008122 Open, NIAID Government Phase 1 24/2/17 Adult 90 Puerto Rico recruiting Inactivated BBV121 Full genome Aluminum No CTRI/2017/05/ Open, Bharat Biotech Industry Phase 1 1/6/17 Adult 48 India whole target 008539 recruiting International organism Ltd., India BIDMC Beth Israel Deaconess Medical Center, NIAID National Institute of Allergy and Infectious Diseases, NIH National Institutes of Health, PIV purified, inactivated whole virus vaccines, ZIKV Zika virus Reference: http://www.who.int/immunization/research/vaccine_pipeline_tracker_spreadsheet/en/ Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 5 of 15 Table 2 WHO – Pipeline Zika virus (ZIKV) vaccines (in preclinical development) (last updated January 2017) January 2017) Platform Candidate Developer/Collaborators Replicating virus Antigen Adjuvant vaccine name (Yes/No) Inactivated whole BK1603 BIKEN No ZIKV full genome To be determined target organism Inactivated whole Bio-Manguinhos in house No ZIKV full genome Alum target organism development and Sanofi Pasteur/WRAIR (discontinued in 2017) Recombinant Bio-Manguinhos No ZIKV E protein Alum subunit (non- VLP) partnership Recombinant Bio-Manguinhos/Aggeu Yes PrM/E and PrM/E/NS1 None viral vector Magalhaes (FIOCRUZ) Live, attenuated Brazilian Ministry of Health Yes rZIKV None recombinant virus agreement with University of Texas rZIKV NS1 Inactivated whole Butantan ZIKV Butantan No ZIKV full genome Alum target organism Live, attenuated Butantan Butantan ZIKV full genome None target organism attenuated ZIKV Inactivated virus + ZIKV Emergent BioSolutions No ZIKV full genome Aluminum aluminum adjuvant Recombinant GEO-ZM05 GeoVax/University of No ZIKV PrM/E + NS1 None viral vector Georgia/CDC Atlanta, US SAM WT GSK-NIH Yes ZIKV prM/E SAM CO GSK-NIH Yes ZIKV prM/E SAM VRC_5283 GSK-NIH Yes ZIKV prM/E SAM VRC_5288 GSK-NIH Yes ZIKV prM/E Recombinant ZIK-80E Hawaii Biotech, Inc. No subunit (non-VLP) Recombinant ZIKVLP Institut Pasteur No subunit VLP Shanghai, China (non-fusion) Recombinant NI.LV-ZIK Institut Pasteur, Paris, No ZIKV prM/E None viral vector France Recombinant ChAdOx1-Zk Jenner Institute ZIKV prM/E None viral vector Inactivated whole KAKETSUKEN ZIKV Kaketsuken No ZIKV full genome TBD target organism Inactivated whole NewLink Genetics No ZIKV prM/E + NS1 TBD target organism Recombinant NewLink Genetics No ZIKV prM-E TBD subunit VLP (fusion) Live, attenuated rZIKV/D2D30 NIAID Yes ZIKV prM/E None target organism Live, attenuated rZIKV/D4D30 NIAID Yes ZIKV prM/E None target organism Live, attenuated rZIKVD30 NIAID Yes ZIKV full genome None target organism Recombinant NIAID Yes None viral vector Sf9 cells/Baculo ZIKV envelope Novavax, Inc. No ZIKV E protein Matrix M adjuvant or dimers (EnvD) aluminum hydroxide Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 6 of 15 Table 2 WHO – Pipeline Zika virus (ZIKV) vaccines (in preclinical development) (last updated January 2017) January 2017) (Continued) Platform Candidate Developer/Collaborators Replicating virus Antigen Adjuvant vaccine name (Yes/No) nanoDNA/ZIKA-LAMP LAMP-ZIKA Pharos Biologicals No ZIKV prM/E chimera construct nanoDNA Recombinant ZIKA recombinant Protein Sciences/Sinergium No ZIKV E protein Aluminum based subunit (non-VLP) Biotech/Lab Liomont Peptide Replikins Zika Replikins Ltd. No Synthetic Peptides None Vaccine and Bocker Recombinant Chimerivax-Zika Sanofi Pasteur ZIKV prM/E + NS1 None viral vector Recombinant SCV-CHIKV+ZIKV Sementis Ltd. No ZIKV, yellow fever, and None viral vector +YF CHIKV surface antigens Recombinant SCV-CHIKV+ZIKV Sementis Ltd. No ZIKV and CHIKV surface None viral vector antigens Recombinant SCV-ZIKV Sementis Ltd. No ZIKV surface antigens None viral vector Inactivated whole Takeda No ZIKV full genome Alum target organism Subunit Tours University No VLP HBV-Zika Prime-boost U1187 INSERM (CYROI, La Reunion) Live attenuated + ZIKV exosome Inactivated whole Valneva No ZIKV full genome Aluminum hydroxide target organism Recombinant viral vector VXA-Zikavax Vaxart ZIKV prM/E VLP VBI-2501A VBI Vaccines No ZIKV E + NS1 DNA WRAIR/BIDMC/Harvard No ZIKV prM/E Recombinant viral vector WRAIR/BIDMC/Harvard ZIKV prM/E BIDMC Beth Israel Deaconess Medical Center, BIKEN Research Foundation for Microbial Diseases of Osaka University, CDC Centers for Disease Control and Prevention, CHIKV Chikungunya virus, GSK GlaxoSmithKline, LAMP loop-mediated isothermal amplification, NIH National Institutes of Health, NIAID National Institute of Allergy and Infectious Diseases, SAM self-amplifying mRNA, SCV Sementis Copenhagen vector, VLP virus-like particles, WRAIR Walter Reed Army Institute of Research, YF Yellow fever, ZIKV Zika virus challenge after a single intradermal immunization [38]or and boosters to sustain protective immunity. ZIKV PIV de- after prime and boost intramuscular immunization [39]. rived from the Puerto Rico strain PRV ABC59 or from The nucleoside-modified mRNA ZIKV vaccine (mRNA- the MR 766 strain, produced in Vero cells, and inacti- 1325), which is being developed by Moderna, a Cambridge- vated with formalin, were tested in either Balb/c mice, based Biotech Company [36], entered a phase 1 clinical trial rhesus monkeys, AG 129 mice, or New Zealand white in December 2016 (NCT03014089). The mRNA candidate rabbits and showed good induction of ZIKV-specific developed by NIAID and GlaxoSmithKline could enter neutralizing antibodies [15, 16, 40]. Further, a ZIKV clinical trials in late 2017. PIV candidate with an alum adjuvant is being evaluated in several phase 1 trials (NCT03008122, NCT02952833, Purified, inactivated whole virus vaccines (PIV) NCT02963909, NCT02937233). The results of three The inactivation process eliminates virus replication phase 1 placebo-controlled, double-blind trials in while maintaining the antigenicity of the structural healthy adults of ZIKV PIV with aluminum hydroxide proteins, and thus PIV are thought to be safe during adjuvant were recently published [41], showing only pregnancy. PIV vaccines have been successfully licensed mild to moderate adverse events. By day 57, 92% of vac- for both JEV and TBEV. ZIKV PIV vaccines would most cine recipients had seroconverted (microneutralization likely be less costly than nucleic acid vaccines. However, titer ≥ 1:10), with peak geometric mean titres seen at day it is plausible that PIVs could require multiple doses in 43 and exceeding protective thresholds seen in animal the primary schedule, adjuvants to enhance immunogenicity, studies. NIAID’s Vaccine Research Center will test a ZIKV Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 7 of 15 PIV as a boost to its DNA Zika vaccine candidate. Bharat replicating virus. Subunit protein and VLP ZIKV vac- and Takeda are also developing a PIV against ZIKV. cines have not yet entered clinical evaluation. Live attenuated vaccines including recombinant Viral-vectored vaccine candidates heterologous flavivirus-vectored vaccines Viral-vectored vaccines share the same ease of pro- Live attenuated vaccines are usually a favored vaccine duction and stability with DNA plasmid vaccines and technology because of their ability to induce durable may therefore be easily scalable in epidemic situa- and effective adaptive immunity at relatively low tions. Viral-vectored vaccines induce both innate and production costs. Live vaccines mimic natural viral adaptive immune responses in mammalian hosts [42]. infections and thus induce a strong antibody and cell- Adenoviral vectors have been used to deliver ZIKV mediated immunity. However, live attenuated vaccines prM-E [40], and were shown to have higher neutral- induce transient low-grade viremia. As CZS is izing antibody titers and T-cell immunity than PIV, thought to occur even in asymptomatically infected DNA, and protein subunit vaccines [15]. Neverthe- pregnant women with low grade viremia [27], repli- less, limitations for adenovirus vaccines include their cating live vaccines need to be carefully evaluated for ability to induce toxic inflammatory responses and their safety prior to their administration to women of the potential for pre-existing immunity to naturally reproductive age, some of whom may be inadvertently occurring human adenoviruses resulting in acceler- pregnant. However, similar to the approach to con- ated clearance and dampened immunogenicity [42]. genital rubella syndrome [44, 45], live attenuated Zika Reactogenicity has been circumvented by the deletion vaccines may play a significant role in endemic trans- of genes required for replication, which also allows mission use, for example, by their incorporation to forlargerinserts [42]. Non-human primate adenovi- childhood vaccination programs in countries with ruses as vaccine vectors can bypass pre-existing im- ZIKV transmission. As ZIKV is a neurotropic virus, munity to human adenoviruses. Adenovirus-vectored neurovirulence and reproductive toxicology testing are and chimpanzee adenovirus-vectored vaccines for critical early steps in the development of live attenu- ZIKV are still in pre-clinical development. ated vaccines prior to human studies. Demonstration The core technology of the measles vector plat- of mosquito non-competence is also required. form developed at the Institut Pasteur in Paris and Live attenuated replication-competent vaccines are now licensed to Themis Bioscience was successfully available for recombinant (or chimeric) flaviviruses. tested in a phase 1 trial for chikungunya virus [43]. The principle of chimerization is to insert target an- The live recombinant measles virus-based chikun- tigens (for example, prM and E) into a back-bone gunya vaccine had good immunogenicity, even in the vector. Sanofi-Pasteur developed a recombinant ZIKV presence of anti-vector immunity, was safe, and had vaccine based on the yellow fever virus 17D back- a generally acceptable tolerability profile, making this bone, which has been used to develop and license the first promising measles virus-based candidate live attenuated recombinant DENV and JEV vaccines vaccine for use in humans. With regards to ZIKV, [46]. NIH/NIAID is also using recombinant DNA the measles vaccine-ZIKV chimeric virus recently en- technology to design recombinant ZIKV/DENV vi- tered a phase 1 clinical trial (NCT02996890). ruses, a strategy employed in the creation of the DENV-2 component of TV003, rDEN2/4Δ30 [47]. Subunit protein/virus-like particles (VLPs) For the ZIKV candidate vaccine, the prM and E cod- Subunit protein vaccines are attractive as a platform ing sequences of ZIKV are being evaluated, replacing due to their potential for safe use in all populations, those of DENV-2 or DENV-4. Combining the NIH including pregnant women, depending on adjuvants. tetravalent DENV vaccine with the recombinant Subunit protein vaccines are produced by transfect- ZIKV/DENV component may provide a combination ing a plasmid encoding a gene sequence of interest DENV-ZIKV vaccine, which could be useful for pop- into bacteria, yeast, or insect cells and utilizing the ulations living in regions endemic for both. machinery within those cells to produce the protein from the gene sequence. Similar to the PIV approach, WHO’s target product profile for a ZIKV vaccine a disadvantage to subunit protein vaccines is that Non-replicating platforms with no documented safety they are generally less immunogenic than live concerns for use during pregnancy would be the pre- vaccines and therefore require multiple doses and ferred vaccine platform for a ZIKV vaccine for emer- adjuvants to achieve protective immunity. The advan- gency use where women of reproductive age are the tage of VLPs is that the antigens are presented in primary target, ideally with a single dose primary their native conformation without the need for a series [6]. Vaccines based on replication-competent Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 8 of 15 platforms are likely to have profiles more suitable for Human immune globulin and anti-ZIKV monoclonal routine/endemic transmission use. As there is a the- antibodies (mAb) for prophylaxis or treatment oretical risk that live, attenuated, or replication- Human immune globulins are used clinically against competent viral vaccines given to pregnant women some viral infections in pregnant women. For measles, may be capable of crossing the placenta and infecting the primary purpose is to attenuate disease in the preg- the fetus [48], live vaccines are generally not recom- nant woman and prevent perinatal transmission to the mended for use during pregnancy. However, live at- newborn. For varicella, the purpose is to prevent or tenuated vaccines have been given to women of attenuate disease in the pregnant woman and prevent child-bearing age (MMR, yellow fever, polio) in situa- congenital infection [50]. However, the incubation time tions of increased risk of exposure, and inadvertent of varicella is 2–3 weeks, far longer than for ZIKV (3– vaccination of pregnant women does occur in mass 10 days), and therefore the critical time to treat is vaccination campaigns. To date, there is no evidence shorter for ZIKV. Plausibly, human immune globulin (or of increased adverse pregnancy outcomes due to hyperimmune globulin) from ZIKV-immune donors, or immunization with a live attenuated vaccine [49]. human mAbs, could be used for prophylaxis or therapy. However, the safety assessment and regulatory re- mAbs are promising because they can be precisely de- quirements for live attenuated/replicating-competent fined and their production controlled and scaled up. ZIKV vaccines are likely to require additional data Blood from a ZIKV-immune donor and a human B-cell compared to non-replicating vaccine platforms. Non- immortalization technique was used to identify human replicating vaccine platforms that either do not use mAbs that bound ZIKV antigens (NS1 and E proteins) any adjuvant or use a well-characterized adjuvant in [51]. An EDIII-specific antibody, ZKA190, protected currently licensed vaccines, such as aluminum salts mice from lethal ZIKV infection, illustrating the poten- (e.g., alum), would be preferable. However, the use of tial for antibody-based therapy. Another mAb, ZIKV- other adjuvants may be justifiable if accompanied 117, was identified as broadly neutralizing of ZIKV with superior performance and delivery aspects (e.g., infection in vitro [52]. Epitope mapping studies have re- reduced number of doses). vealed that ZIKV-117 recognized a unique quaternary epitope on the E protein dimer-dimer interface. Treat- Zika therapeutics ment of Zika-infected pregnant and non-pregnant mice Therapeutics against ZIKV need to be developed in with ZIKV-117 markedly reduced tissue pathology, pla- parallel to vaccines and may have a specific role in cental and fetal infection, and mortality. A bispecific reducing the burden of Zika infection and disease in mAb has also been developed that could address con- the populations most at risk of serious outcomes. cerns about the emergence of anti-viral resistance to Drugs could rationally be used for prophylaxis or monospecific mAbs [53]. Collectively, these data demon- post-exposure prophylaxis to prevent or mitigate the strate the feasibility of developing mAbs as therapeutic severity of CZS, and may have particular value when and/or prophylactic candidates. low endemicity does not justify widespread immunization. Aborting ongoing ZIKV shedding in Small molecule antivirals for prophylaxis or treatment seminal fluids may be another indication. Antivirals Multiple studies have demonstrated the anti-ZIKV activ- are the cornerstone of management of chronic hu- ity of several Food and Drug Administration (FDA)-ap- man viral infections like HIV, hepatitis B, and hepa- proved drugs or drug candidates being clinically tested titis C. There are also precedents for therapies to for other indications [54–59]. For example, the anti- manage viral infection in pregnant women and their HCV prodrug Sofosbuvir has anti-Zika virus activity in fetussuch aspost-exposure prophylaxiswith immune vitro [54]; however, repurposing this compound is prob- immunoglobulins in susceptible women to protect lematic because its hydrolysis is highly specific to the the mother and fetus from infection with varicella. liver. Niclosamide, a category B anthelmintic drug, Anynew drugsfor ZIKV wouldbeusedasanad- inhibited ZIKV replication at low micromolar concentra- junct to the standard of care for non-pregnant and tions [58]. However, the poor systemic bioavailability of pregnant persons, and may be indicated before vac- niclosamide is a hurdle to further clinical development cines become widely available or in addition to vac- against Zika. More than 20 out of 774 FDA-approved cine programs. drugs decreased ZIKV infection in an in vitro screening Three plausible clinical indications for application of a assay [54]. Selected compounds were further validated for medicinal prophylactic/therapeutic against ZIKV are (1) inhibition of ZIKV infection in human cervical, placental, to offer prophylaxis or early post-exposure prophylaxis, and neural stem cell lines, as well as in primary human (2) to accelerate viral clearance, and (3) to reduce amnion cells. Established anti-flaviviral drugs (e.g., borte- disease severity (Box 1). zomib and mycophenolic acid) and others with no Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 9 of 15 Box 1: Clinical indications for application of a prophylactic/therapeutic against Zika INDICATION 1. PROPHYLAXIS OR EARLY POST-EXPOSURE PROPHYLAXIS. (1) To prevent maternal infection and fetal disease: The objective is to prevent, or diminish, Zika virus (ZIKV) infection and disease in pregnant women, or women trying to become pregnant, and thus eliminate or substantially reduce the probability of intrauterine infection or transmission in the perinatal or postnatal period. Examples of this approach for Zika. Prophylaxis for a pregnant woman living in a setting where there is epidemic Zika transmission, or a pregnant traveler spending time in a Zika-affected location. Post-exposure prophylaxis might be considered where there is strong suspicion that a pregnant woman has been exposed to Zika because of epidemiological circumstances (e.g., lives in an endemic area and resides in a household where recent Zika cases have been diagnosed) Challenges. The risk/benefit of using prophylactic agents (small molecule drugs or immune globulin) must be balanced against the probability of the mother and fetus being infected and of that infection harming the fetus Examples of this approach for other infectious diseases. Varicella virus (chickenpox) infection during pregnancy may result in congenital varicella, which is usually benign and self-limiting, but can occasionally produce a characteristically severe pattern of abnormalities known as ‘congenital varicella syndrome’. Zoster immunoglobulin (a preparation of human Ig containing anti-varicella antibodies) is indicated for all pregnant women who have significant exposure to Varicella Zoster virus infection (defined as ‘living in the same household as a person with active chickenpox or herpes zoster or face-to-face contact with a person with chickenpox or uncovered zoster for at least 5 minutes’), who have no history of chickenpox and who are seronegative. (2) To prevent Guillain–Barre Syndrome or other ZIKV infection-related neurological complications. Prophylaxis throughout the duration of exposure (e.g., travel to a Zika endemic area). Early post-exposure prophylaxis after known exposure to a Zika case (e.g., sexual exposure, nosocomial exposure such as needle stick injury, living in same household of a current Zika case). INDICATION 2. THERAPY TO ACCELERATE RESOLUTION OF INFECTION Examples of this approach for Zika. A pregnant woman, who lives in a setting where ZIKV is known to circulate, presents to a clinic with clinical signs and symptoms that could represent ZIKV infection. Out of an abundance of caution, empiric treatment commences before the results of laboratory tests are known (if such tests are available). The purpose of treatment is to accelerate clearance of virus infection from the maternal tissues and mitigate the likelihood of intrauterine or peri- or postnatal virus infection. A second example of treatment is for neonates who have acquired ZIKV from intrauterine exposure or perinatally. Challenges. As for prophylaxis, the use of a therapeutic agent must be balanced by the safety and cost profile of the drug and the likelihood that treatment will deliver clinical benefits to the fetus, i.e., prevent or modify intrauterine infection. For many Zika cases, viremia is already in rapid decline, or even undetectable by the time the patient presents to healthcare providers. An additional delay is created if treatment is guided by laboratory diagnostics rather than an empiric approach. Examples of this approach for other infectious diseases. There are no examples for acute viral infections, but in the setting of chronic infections such as HIV, it is well accepted that vertical transmission is RNA copy number-dependent, with higher rates occurring with increasing viral loads present in the mother. Treatment of pregnant women has been demonstrated to drastically reduce the incidence of vertical transmission in women undergoing treatment with a combination of antiretroviral compounds [82]. INDICATION 3. DISEASE MODIFICATION FOR EXISTING CONGENITAL INFECTION. The objective is the treatment of existing intrauterine fetal infection by eradication of virus and thus reduce the severity of congenital Zika syndrome. Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 10 of 15 Examples of this approach for Zika. A pregnant woman has amniocentesis performed because of concerns about recent exposure to ZIKV. The amniocentesis fluid is RT-PCR positive for ZIKV. Treatment is commenced to eradicate virus from the fetal tissues. Challenges. This indication is unlikely to be the sole basis for drug development. Clinical trials to test for improvements in fetal outcome would be very long in duration and likely prohibitively expensive to perform. The extent of viremia in the pregnant woman before commencement of treatment may already have led to significant pathology of the fetus. Examples of this approach for other infectious diseases. Unfortunately, there is no evidence that the outcome of an established congenital viral infection (e.g., with cytomegalovirus or varicella) can me modified by small molecule drug or immunoglobulin treatment. previously known antiviral activity (e.g., daptomycin) were the same model (Cristina Cassetti; personal communica- identified as inhibitors of ZIKV infection. These results tion). A summary of compounds found to have Zika anti- offer the possibility of a repurposed drug being used for viral properties in vitro (Table 3) and of some of the Zika therapeutic or prophylactic indications. repurposed drugs reported to have anti-Zika activities are Newly discovered candidate anti-virals include a synthetic provided herein (Table 4). peptide derived from the stem region of the ZIKV envelope protein, designated Z2, which potently inhibits infection of Challenges for clinical evaluation of Zika vaccines ZIKV and other flaviviruses in vitro [60]. Z2 is able to pene- and therapeutics trate the placental barrier to enter fetal tissues and prevent Various challenges may delay or hinder the successful licen- vertical transmission of ZIKV in pregnant C57BL/6 mice sure of Zika vaccines or therapeutics, as described below. [60]. Another molecule, galidesivir, is an adenosine analogue active in cell culture against a wide-range of RNA Selection of the most suitable clinical endpoint viruses [61]. Galidesivir treatment of ZIKV-infected mice In June 2017, WHO convened a meeting to elaborate on significantly improved survival even when treatment was clinical endpoints for ZIKV vaccine efficacy trials [10]. Al- initiated 5 days after infection [62]. However, potential hur- though preventing CZS is the outcome of greatest interest dles for galidesivir development is the requirement for an for public health, the large sample sizes required, the focus oral formulation (galidesivir requires parenteral administra- on women only, the heterogeneity of clinical manifesta- tion). Ribavirin, another broad-spectrum but teratogenic tions of CZS, and ethical considerations render CZS as antiviral, did not improve outcomes from ZIKV infection in the primary endpoint unfeasible. A possible endpoint for Table 3 List of potential compounds for repurposing with anti-Zika activity, extracted from [19, 83] Drug group Drug name Description Nucleoside analogs Sofosbuvir, MK-608 • Inhibit Zika virus (ZIKV) replication in cellular assays • Efficacious in animal models 2CMC, Ribavirin, • Showed antiviral activity in cell culture Favipiravir, T1105 BCX4430, GS5734 • Reduced mortality in ZIKV-infected mice • Currently in phase I and II clinical trials Peptidomimetic agents CN-716 • Inhibit ZIKV protease in vitro, but only weakly inhibit viral replication • Due to safety reasons, may not translate as therapeutic option for pregnant women, but could be applied to other infected individuals Adenosine analog NITD008 • Showed potent anti-ZIKV activity • Could serve as a reference inhibitor for future drug screen and discovery Cyclin-dependent PHA-690509 • Showed inhibition of ZIKV replication of all three strains kinase inhibitor Antimalaria Chloroquine • Reduces virus production, the number of infected cells, and cell death promoted by ZIKV infection without any cytotoxic effect • Promising candidate for ZIKV clinical trials • Can be safely administered to pregnant women since it is clinically approved Anthelmintic Bithionol • Propagate by activating host caspases and inducing programmed cell death Epigallocatechin gallate • Natural compound found in food items, particularly green tea • It inhibits ZIKV entry into host cell Interferon-inducible • Inhibit the replication of a number of pathogenic viruses transmembrane proteins Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 11 of 15 Table 4 High throughput screening for potential compounds with anti-Zika activity (drug repurposing) Study No. of drugs Compounds identified with anti-Zika activity Remarks screened Barrows et al. [54] 774 FDA-approved Clofazimine, Digoxin, Gemcitabine, Ivermectin, • Mycophenolic acid (MPA), Ivermectin, agents Mefloquine, Mercaptopurine hydrate, Mycophenolic Daptomycin, Mefloquine, Palonosetron acid, Fingolimod, Mycophenolate mofetil, Dactinomycin, identified as having higher potency Bortezomib, Methoxsalen (Xanthotoxin), Azathioprine, • Daptomycin, Mefloquine, and Palonosetron are Thioguanine, Auranofin, Sertraline, Pyrimethamine, pregnancy category B drugs Daptomycin, Palonosetron, Deferasirox, Micafungin, Sorafenib tosylate, Cyclosporine A, Mebendazole Xu et al. [58] 6000 compounds, Niclosamide, Emricasan, 10 structurally unrelated • Emricasan is an inhibitor of caspase-3 activity > 2000 inhibitors of CDK but uncertain if a capase-3 inhibitor with FDA-approved agents anti-inflammatory properties impacts development of unborn fetus clinical trials could be ZIKV infection (whether symptom- and ensuring that pregnant women have fair access to atic or not), which would require a smaller sample size participate in ZIKV vaccine trials that offer a favorable compared to a clinical endpoint. However, detecting ratio of risks to potential benefits. Clinical development asymptomatic ZIKV infections (as measured by serocon- plans should therefore include systematic collection of version or sampling for virological detection) poses several relevant indicators and outcomes of safety and efficacy challenges, including the requirement of very frequent for pregnant women. Although certainly a complex chal- blood, urine, and possibly semen collection so as not to lenge, a concerted and proactive effort is required to ad- miss the acute infection and achieve virological diagnosis dress the needs of pregnant women and their offspring [63]. Vaccination may also interfere with serological test- early and across the ZIKV vaccine R&D pathway. ing, e.g., it may render it difficult to discriminate between vaccine response and natural infection. A challenge with Sample size and trial site selections using clinical disease as the primary endpoint is that ZIKV Generating clinical efficacy data in a reasonable sample illness is often associated with mild and non-specific size and an acceptable timeframe and cost is challenging symptoms, which raises challenges for case detection. A at a time when global Zika incidence has declined to low standardized clinical case definition is essential to facilitate levels. Areas with recent active ZIKV transmission may the comparison and combining of information from dif- not be the best sites for clinical trials. Given that estimates ferent studies. A working case definition of virologically of ZIKV seroprevalence are as high as 70% in some areas confirmed Zika illness has been provided by the Pan that experienced an outbreak, the proportion of suscep- American Health Organization [64]. tible individuals in such populations will be low, with a The consensus at the WHO technical consultation in subsequent incidence too low to sustain an efficacy trial. June 2017 was to select virologically confirmed clinical ill- Therefore, the WHO technical consultation in June 2017 ness as the primary endpoint, and to additionally study a proposed the projection of future evolution of the ZIKV subset to explore the protection against infection or reduc- epidemic based on the presence and vectorial capacity of tion in viremia. The underlying assumption is that reduc- Aedes mosquitoes [66, 67], travel patterns [68–70], and tion in ZIKV disease incidence is associated with either risk mapping and modeling [71–74] to predict the move- sterilizing immunity or a reduction in ZIKV viremia, which ment of Zika [75, 76]; various mathematical modeling in turn will reduce or prevent subsequent development of groups are working to this end. A multi-site approach for complications in pregnant and non-pregnant individuals. vaccine trials will be needed to increase the chance of in- cluding populations with a high incidence of disease, as Inclusion of pregnant women in trial design and safety well as providing an opportunity to evaluate vaccine effi- considerations cacy across different populations. Although pregnant women would not be the primary target population for efficacy trials based upon the above rationale, pregnant women remain a priority population Immune correlates for ZIKV vaccine use in areas experiencing ongoing An immune correlate of protection is an immune transmission and in future outbreaks. Thus, the Ethics response marker that is statistically associated with pro- Working Group on ZIKV Research and Pregnancy [65] tection from disease or infection and may be either recommended the collection of data specific to safety mechanistic (causally related to outcome) or non- and immunogenicity in pregnancy for all ZIKV vaccine mechanistic/surrogate (statistically related to outcome). candidates to which pregnant women may be exposed Given the global decline in cases, it is unclear whether Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 12 of 15 large scale efficacy trials are viable given the current in- for at least 3–5years from thetimeofcompletionofpri- cidence of ZIKV transmission. If clinical efficacy trials mary vaccination due to the concern of immune enhance- are not feasible, immune correlates/surrogates derived ment [80]; however, given the lack of data supporting a from passive protection studies in animals, natural his- clinical significant interaction between DENV and ZIKV tory studies, and controlled human challenge study re- [26–28], such a formal recommendation has not yet been sults may possibly represent acceptable endpoint data made for Zika vaccine development. Nevertheless, a longer for initial emergency use authorization and eventual follow-up period to monitor safety could be considered. licensure. ‘Accelerated approval’ is based on the demon- stration of a surrogate of protection though well- Establishing a transparent framework for selecting vaccines controlled clinical studies that are reasonably likely to Given the global decline in ZIKV incidence and the po- predict clinical benefit. The US FDA ‘animal rule’ is tential bottleneck in identifying suitable trial sites, a pro- based on the demonstration of an immune marker of posal was made during the June 2017 WHO technical protection in animal models that will reasonably likely consultation to establish a transparent framework for predict clinical benefits in humans. Both accelerated ap- prioritizing vaccines to be evaluated in phase 2b/3 trials. proval and animal rule approaches require post- Selection criteria would depend on the desired attri- licensure studies to verify clinical benefit and safety. butes, including compliance with the target product pro- Controlled human infection models are a promising av- file, pre-clinical evidence of complete or near-complete enue to explore immune correlates in humans, however, prevention or reduction of viremia, safety during preg- they are associated with complex ethical considerations. nancy, and scalability of the product. The feasibility of establishing immune correlates or sur- rogates is now a priority. Donor and industry fatigue Major vaccine producers, government-funded insti- Assay optimization and standardization tutions, academics, and small to mid-size research A comprehensive review of ZIKV diagnostics was recently enterprises responded promptly to the Zika out- performed [77] and shortcomings highlighted [63]. In the break, setting aside other activities to focus on rap- context of a highly epidemic disease with an apparent short idly developing vaccines and therapeutics against duration of detectable viremia and relatively infrequent inci- Zika, supported by government and philanthropic dence of clinical disease, reliable case ascertainment in effi- funding agencies. However, with the rapid decline in cacy trials is critical. However, the short and relatively low cases, the unpredictability of future outbreaks, and level viremia is difficult to detect, and the serological assays the still poorly defined use scenarios, the commer- lack specificity because of cross-reactivity between other co- cial market has become questionable. The prospect circulating flaviviruses and flavivirus vaccines [78]. Frequent of a licensed Zika vaccine is at stake unless govern- sampling over time and sampling of various bodily fluids ments and other donors sustain the level of support (whole blood, serum, urine), as well as the combination of to advance development. Current models for stimu- various diagnostic assays will be necessary to increase the lating epidemic product development are failing. The diagnostic yield. For the comparability of clinical trial re- Coalition for Epidemic Preparedness Innovations sults, it is crucial to standardize diagnostic assays used (CEPI) is a new alliance between governments, in- and immunological reference reagents should be available. dustry, academia, philanthropy, intergovernmental The plaque reduction neutralization test is still considered institutions (such as WHO), and civil society, and to be the laboratory standard against which other neutral- was founded to finance and coordinate the develop- izing antibody assays should be compared. A guideline on ment of new vaccines to prevent and contain infec- plaque reduction neutralization test standardization can tious disease epidemics [81]. Zika is not yet on the be found on the specific WHO website [79]. priority list for CEPI, but as donor and industry fa- tigue may increase, CEPI, or such other mecha- Interaction between DENV and ZIKV nisms, will be needed to ensure that, out of the Given the widespread endemicity of DENV in the areas many Zika vaccine candidates, at least one will make most affected by the current ZIKV outbreak, and the fact it to the finish line. that short- or long-term immunological interaction be- tween DENV and ZIKV cannot currently be excluded, trials Conclusion would ideally need to take baseline blood samples for all At least 45 Zika vaccine candidates have been or are in subjects to ascertain prior DENV exposure in order to development, some of them already in phase 2 clinical study the impact of prior immunity to DENV on vaccine trials. Multiple vaccine platforms have shown robust performance and safety. For DENV vaccines, WHO recom- protection against ZIKV challenge in animal models. mends that subjects are followed-up for safety and efficacy However, unique challenges will need to be addressed in Wilder-Smith et al. BMC Medicine (2018) 16:84 Page 13 of 15 the clinical development and regulatory pathways of a Ethics approval and consent to participate Not applicable. ZIKV vaccine that may hinder the development, licen- sure, and WHO-prequalification of high-quality, safe, Competing interests and effective ZIKV vaccines. Implementing phase 3 effi- JH is an employee of the World Health Organization (WHO), as was KV at the cacy trials will be difficult given the challenges of the time of this work. AWS is consultant to WHO. The authors alone are responsible for the views expressed in this publication and they do not spatial and temporal heterogeneity of ZIKV transmis- necessarily represent the decisions or policies of WHO. The authors declare sion, the unpredictability of the ZIKV epidemics, the no conflicts of interest. broad spectrum of clinical manifestations making a sin- gle definite endpoint difficult, the lack of sensitive and Publisher’sNote specific diagnostic assays, and the need for inclusion of Springer Nature remains neutral with regard to jurisdictional claims in vulnerable target populations. In addition to a vaccine, published maps and institutional affiliations. drugs for primary prophylaxis, post-exposure prophylaxis, Author details or treatment should also be developed in order to prevent 1 Immunization, Vaccines & Biologicals, World Health Organization, Geneva, or mitigate the severity of CZS. The global research and Switzerland. Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore, Singapore. Department of Epidemiology and Global public health community should prioritize the develop- Health, Umea University, Umea, Sweden. Center for Immunization Research, ment of ZIKV vaccines and therapeutics that will be ac- Johns Hopkins Bloomberg School of Public Health, Baltimore, MD, USA. ceptable for use by women of reproductive age, and ensure State University of New York, Upstate Medical University, Syracuse, NY, USA. Doherty Institute for Infection and Immunity, Parkville, VIC 3010, Australia. availability and affordability for use in countries where 7 8 The Royal Melbourne Hospital, Parkville, VIC 3010, Australia. Oxford ZIKV is circulating. To this end, WHO is working towards University Clinical Research Unit, 764 Vo Van Kiet street, District 5, Ho Chi a roadmap for Zika vaccine and product development. Minh City, Vietnam. Institute of Vector-borne Disease, Monash University, Melbourne, VIC, Australia. Abbreviations Received: 30 October 2017 Accepted: 1 May 2018 CEPI: Coalition for Epidemic Preparedness Innovations; CZS: congenital Zika syndrome; DENV: Dengue virus; FDA: Food and Drug Administration; GBS: Guillain–Barre syndrome; JEV: Japanese encephalitis virus; mAb: monoclonal antibodies; mRNA: messenger RNA; NIH: National Institutes References of Health; PIV: purified, inactivated whole virus vaccines; R&D: research and 1. Heymann DL, Hodgson A, Sall AA, Freedman DO, Staples JE, Althabe F, development; TBEV: tick-borne encephalitis virus; VLP: virus-like particles; Baruah K, Mahmud G, Kandun N, Vasconcelos PF, et al. Zika virus and WHO: World Health Organization; ZIKV: Zika virus microcephaly: why is this situation a PHEIC? Lancet. 2016;387(10020):719–21. 2. World Health Organization. WHO and Experts Prioritize Vaccines, Diagnostics and Innovative Vector Control Tools for Zika R&D. http://www.who.int/mediacentre/ Acknowledgements news/notes/2016/research-development-zika/en. Accessed Jan 2018. We would like to thank Vasee Moorthy (WHO) and Farah Al-Shorbaji (WHO) 3. Durbin AP. Vaccine development for Zika Virus – timelines and strategies. for their help with the WHO Zika vaccine pipeline tracker, and Wei-Yee Semin Reprod Med. 2016;34(5):299–304. Leong, Lee Kong Chian School of Medicine, Singapore, for her secretarial 4. Durbin A, Wilder-Smith A. An update on Zika vaccine developments. Expert support. We would like to acknowledge the following persons for sharing Rev Vaccines. 2017;16(8):781–7. information for the WHO pipeline tracker: Dave Anderson (VBI), Sudhakar 5. World Health Organization. WHO Zika Virus (ZIKV) Vaccine Target Product Bangera (Bharat Biotech), Anne Bogoch Borsanyi (Replikins), Scott Butler Profile (TPP): Vaccine to Protect Against Congenital Zika Virus Syndrome for (Moderna), James Cummings (Novavax), Marcos da Silva Freire Use During an Emergency. Geneva: WHO/UNICEF; 2016. (Bio-Manguinhos), Shailesh Dewasthaly (Valneva), Christiane Gerke (Institut 6. World Health Organization. WHO Zika virus and complications: 2016 Public Pasteur), Farshad Guirakhoo (GeoVax), Paul Howley (Sementis), Nicholas Health Emergency of International Concern. http://www.who.int/ Jackson (Sanofi Pasteur), Fernando Lobos (Sinergium Biotech), Joel emergencies/zika-virus/en/. Accessed Jan 2018. Maslow (GeneOne), Kayvon Modjarrad (WRAIR), Tom Monath (NewLink), 7. World Health Organization. WHO R&D Blueprint. 2016. www.who.int/ Kensuke Nakajima (Japan Agency for Medical Research and Development blueprint/about/en/. Accessed Apr 2018. (AMED)), Alexander Precioso (Butantan), Arturo Reyes-Sandoval (Jenner 8. Vannice KS, Giersing BK, Kaslow DC, Griffiths E, Meyer H, Barrett A, Durbin AP, Institute), Barbara Solow (Emergent Biosystems), Erich Tauber (Themis Wood D, Hombach J. Meeting Report: WHO consultation on considerations for Bioscience), J. 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Geneva: WHO; 2016. http://www.who.int/blueprint/ innovation programme under ZikaPLAN (Grant Agreement No. 734584), and priority-diseases/key-action/global_consultation_of_research_related_to_ the Lee Kong Chian School of Medicine start up grant. zika.pdf?ua=1. Accessed Jan 2018 11. Dowd KA, DeMaso CR, Pelc RS, Speer SD, Smith AR, Goo L, Platt DJ, Mascola Authors’ contributions JR, Graham BS, Mulligan MJ, et al. Broadly neutralizing activity of Zika virus- AWS wrote the first and final draft; KV was in charge of the WHO pipeline immune sera identifies a single viral serotype. Cell Rep. 2016;16(6):1485–91. tracker for ZIKV vaccines and developed Tables 1 and 2; CPS and IT wrote 12. Aliota MT, Dudley DM, Newman CM, Mohr EL, Gellerup DD, Breitbach ME, the text on therapeutics, CPS and IT created Box 1, AWS created Tables 3 Buechler CR, Rasheed MN, Mohns MS, Weiler AM, et al. 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