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Monday, 18 October 2004 - 8:45 AM

This presentation is part of: Opening Plenary Session (6)

PREDICTING THE IMPACT OF A PARTIALLY EFFECTIVE HIV VACCINE AND SUBSEQUENT RISK BEHAVIOR CHANGE ON THE HIV EPIDEMIC IN DEVELOPING COUNTRIES: A SOUTH AFRICAN EXAMPLE

Kyeen Mesesan, BS, Yale University School of Medicine, Department of Epidemiology & Public Health, Division of Epidemiology of Microbial Diseases, New Haven, CT, Douglas K. Owens, MD, MS, VA Palo Alto Health Care System, Center for Primary Care and Outcomes Research, Stanford University, Palo Alto, CA, and A. David Paltiel, PhD, Yale University School of Medicine, Department of Epidemiology & Public Health, Division of Health Policy & Administration, New Haven, CT.

Purpose: To assess the impact of partially effective HIV vaccines, and subsequent changes in behavior, in a population at high risk for heterosexually transmitted HIV.

Methods: We developed a dynamic, compartmental epidemic model to simulate the impact of various low-efficacy preventive HIV vaccination scenarios in Soweto, South Africa. We adapted a previously published model [Owens, 1998 & Edwards, 1998] of HIV vaccines in a homosexual male population in San Francisco. Differential equations were estimated to govern transitions between population subgroups defined by sex, disease stage, and vaccination status. We explored changes in vaccine efficacy and post-vaccination condom use. Assumptions included (1) initial HIV prevalence of 31%, (2) an anti-retroviral-naïve population, (3) exclusive male negotiation of condom use, and (4) contact rates of 0-3 partners/yr and per-partner infectivity rates of 0.054-0.228, depending upon both disease stage and male-female vs. female-male transmission.

Results: With no change in risk behavior, a 40% effective HIV vaccine would avert 181,000 infections in Soweto over a span of 10 years and reduce future HIV prevalence from 32% to 23%. A 30% effective vaccine would avert 141,000 infections and reduce HIV prevalence to 25%. A 20% effective vaccine would avert 99,000 infections and reduce HIV prevalence to 27%. Changes in risk behavior would have a significant impact on the efficacy of these vaccination programs. With a 40% effective vaccine, a 25% increase in condom use among vaccinated individuals would instead avert 223,000 infections and reduce the HIV prevalence to 21%. However, if condom use decreased by 25%, a 40% effective HIV vaccine would only avert 136,000 infections and reduce the HIV prevalence to 25%. Some combinations of vaccine efficacy and risk behavior change could even worsen the epidemic. A 30% effective HIV vaccine with a resulting 75% decrease in condom use, for example, would cause an additional 29,000 infections and increase the HIV prevalence to 33% over the 10-year period.

Conclusions: Even modestly effective HIV vaccines can confer enormous benefits in terms of HIV infections averted and decreased HIV prevalence. However, these findings are sensitive to assumptions regarding the impact of vaccination on subsequent risk behavior. For South Africa and other developing countries with similar epidemic profiles, programs to reduce risk behavior may be important components of successful vaccination programs.


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