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Pasqualucci, Cristina; Ravà, Lucilla; Rossi, Carla; Schinaia, Giuseppe
doi: 10.1177/003754979807100402pmid: N/A
The HIV/AIDS epidemic has reached the end of its second decade, and estimations of its size and dynam ics are very important, particularly for developing and planning prevention and therapeutic interventions. Since the HIV infection is characterized by a long asymptomatic period and latency time, observational studies are not suitable to study the epidemic. Dy namic models and the Back-Calculation (BC) methods are two general methodologies which provide estimates and projections of HIV/AIDS incidence and prevalence, while focusing attention on different aspects of the epidemic. For example, the Mover-Stayer (MS) Model, a dynamic compartmental model developed to study the epidemic's spread among a general population, is a useful tool to perform scenario analysis, while the Empirical Bayesian Back-Calculation (EBBC) method allows investigation of the dynamics of the epidemic by risk category. Both methods, with timely updating to reflect changes in the natural history of the HIV/ AIDS epidemic, were recently applied within the EU Concerted Action on Multinational AIDS Scenario Analysis and the Italian Consensus Conference. An overview of some dynamic models and BC methods, and an application of the MS Model and EBBC method to the Italian HIV/AIDS epidemic, are presented here.
Adams, Andrew L.; Barth-Jones, Daniel C.; Chick, Stephen E.; Koopman, James S.
doi: 10.1177/003754979807100403pmid: N/A
Many HIV vaccine trials have been proposed to evaluate susceptibility of individuals. However, vac cines may also affect an epidemic's course at the population level by altering the infectiousness of vaccinated individuals who become infected. A vac cine trial design that does not estimate both suscep tibility and infectiousness might reject a proposed vaccine that is capable of halting the HIV epidemic. We describe a vaccine trial design called the Retro spective Partner Trial (RPT), which can quantify vaccine effects on both susceptibility and infectious ness. We describe HIVSIM, a simulation environ ment that generates simulated populations and al lows for empirical evaluation of the statistical power of the RPT. HIVSIM explicitly models a number of factors which influence transmission and preva lence, and which have proven difficult to model us ing standard models. These factors include the infec tion stage of infected individuals, partnership selec tion, the duration of partnerships and concurrence, and transmission of HIV. The simulation analysis indicates that the RPT design has substantially greater statistical power for identifying vaccines which, in spite of exhibiting poor protection against infection, are nonetheless capable of halting the HIV epidemic by substantially reducing the infectious ness of vaccinated individuals who become infected.
Welch, Gavin; Chick, Stephen E.; Koopman, James
doi: 10.1177/003754979807100404pmid: N/A
The disease gonorrhea (GC) is a major public health problem in the United States, and the dynamics of the spread of GC through popula tions are complicated and not well understood. Studies have drawn attention to the effect of concurrent sexual partnerships as an influen tial factor for determining disease prevalence. However, little has been done to date to quantify the combined effects of concurrency and within-partnership sex-act rates on the prevalence of GC. This simulation study examines this issue with a simplified model of GC transmission in closed human populations that include concurrent partnerships. Two models of within-partnership sex-act rate are compared; one is a fixed sex-act rate per partnership, and the other is perhaps more realistic in that the rate depends on the number of concurrent partners. After controlling for total number of sex acts, pseudo-equilibrium prevalence is higher with the fixed sex-act rate than under the concurrency-adjusted rate in all the modeled partnership formation conditions.
McKenzie, F. Ellis; Wong, Roger C.; Bossert, William H.
doi: 10.1177/003754979807100405pmid: 18668185
We develop discrete-event simulation models using a single "timeline" variable to repre sent the Plasmodium falciparum lifecycle in individual hosts and vectors within interacting host and vector populations. Where they are comparable, our conclusions regarding the relative importance of vector mortality and the durations of host immunity and parasite development are congruent with those of classic differential-equation models of malaria epidemiology. However, our results also imply that in regions with intense perennial transmission, the influence of mosquito mortality on malaria prevalence in humans may be rivaled by that of the dura tion of host infectivity.
Zaric, Gregory S.; Brandeau, Margaret L.; Bayoumi, Ahmed M.; Owens, Douglas K.
doi: 10.1177/003754979807100406pmid: N/A
Recent advances in the development of protease inhibi tors offer the promise of significantly longer lives for many HIV-infected individuals. However, the drug dosing regimens are often difficult for patients to follow, and non-compliance can enhance the development of drug-resistant strains of HIV. This paper uses com puter simulation to analyze the effects of such "multi- drug therapy" and treatment compliance on the spread of HIV and of multi-drug-resistant HIV strains. The analysis is based on a dynamic compartmental model of the epidemic simulated using Excel, a spreadsheet package. Our analysis shows that multi-drug therapy can lead to significantly high prevalence of multi-drug- resistant HIV strains. Even under optimistic assump tions about rates of compliance and rates at which drug resistance develops, the development of multi-drug- resistant HIV strains in the population may be sig nificant. Our analysis shows that the most important factors affecting the development of multi-drug-resis tant HIV strains are initial and continuing compli ance with treatment. New treatments that are easier to comply with or behavioral interventions that increase patients' compliance could significantly reduce the spread of multi-drug-resistant HIV strains.
Anderson, James G.; Anderson, Marilyn M.
doi: 10.1177/003754979807100407pmid: N/A
Clinical trials have shown that HIV-positive preg nant women treated with zidovudine during preg nancy and delivery have a much lower rate of HIV transmission to their newborns. Recent studies have also shown the effectiveness of antiretroviral therapy combined with elective cesarean delivery and a short- term antiretroviral therapy. In this study a dynamic computer model was built. The model was used to estimate economic consequences of testing pregnant women for HIV virus and treating the HIV-positive women and their newborns with three different reg imens. Outcomes of data analysis included costs and the number of avoided cases of perinatal transmission of the HIV virus. The study found that while the over all costs of the three treatment programs were simi lar, the short-term antiretroviral program was the least cost-effective and prevented fewer cases of peri natal transmission of the virus. Sensitivity analysis found that the cost-effectiveness of the treatment pro grams were differentially sensitive to screening and treatment rates. Cost-effectiveness of all regimens is highly sensitive to the HIV infection rate among preg nant women. When the rate reaches four to five per 1,000, treatment cost savings from the reduction in pediatric HIVcases exceed the costs of screening and treatment. The model presented provides a useful tool to be used to evaluate treatment programs designed to pre vent mother-to-infant transmission of the HIV virus.
doi: 10.1177/003754979807100409pmid: N/A
Last summer there was a 50th Anniver sary Celebration of the birth of the "first stored program computer" at the Univer sity of Manchester, "The Home of Com puting," in Manchster, UK. As a part of that celebration, The Society for Com puter Simulation International organized the European Simulation Multiconference 1998, and this Editor was in vited to participate.When logistics prevented my attendance it was sug gested that I prepare a videotape as my contribution. With the aid of a friend who furnished equipment and acted as interviewer, we made a very amateurish videotape.In lieu of being able to show it to you here, a slightly modified text follows—JM
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