A Dynamic HIV-Transmission Model for Evaluating the Costs and Benefits of Vaccine Programs

Abstract

We developed a dynamic model of HIV transmission to evaluate the costs and benefits of HIV-vaccine programs in a population of homosexual men. We examined how changes in high-risk sexual behavior and the growth pattern of the epidemic influence the cost-effectiveness of preventive vaccines and of therapeutic vaccines. We found that the effect of reductions in condom use is more important for therapeutic vaccines than for preventive vaccines. Therapeutic vaccines may increase HIV seroprevalence in the population, unless the vaccine program is accompanied by increased condom use. Epidemic growth patterns also influence the cost-effectiveness of both vaccines, but the effects are more pronounced for preventive vaccines, which are more cost-effective in an early-stage epidemic than in a late-stage epidemic.

Predicting the Impact of a Partially Effective HIV Vaccine and Subsequent Risk Behavior Change on the Heterosexual HIV Epidemic in Low- and Middle-Income Countries

“…18,23,25 To our knowledge, this also represents the first time that differential condom use behavior between men and women has been incorporated into a model of HIV disease transmission. Although Western models of HIV transmission have specified that condom use between 2 partners with differing practices is the greater of the 2 (eg, if one person uses condoms 70% of the time and another person uses condoms 30% of the time, condom use in that particular partnership is 70%), 25 we believe that this does not accurately reflect condom use negotiation in the African setting. We assumed exclusive male negotiation of condom use in our model to reflect the decreased ability of South African women to negotiate condom use in heterosexual partnerships, 60-63 a disadvantage attributable to cultural and economic barriers that is found in many countries throughout sub-Saharan Africa and, in part, explains the shift toward a female-dominated epidemic.…”

“…[9][10][11][12][13][14] Models have shown that even a partially effective HIV vaccine with minimal efficacy could have an enormous population impact on the HIV epidemic in high-risk groups or areas with a high prevalence of HIV, such as sub-Saharan Africa, [15][16][17][18][19][20][21][22] but that the magnitude of this impact depends on a fine balance between the efficacy of the vaccine, the vaccine coverage level, and the change in risky behavior. 18,20,[23][24][25][26][27] …”

“…19,25,48,49 For our base-case scenario, we used a 40% effective vaccine (ε = 0.4), a figure that was subsequently varied. We simulated a program in which vaccines were given to unvaccinated asymptomatic men and women aged 17 years or older without testing individuals for HIV; therefore, vaccines were given to uninfected and asymptomatic HIV-positive individuals.…”

“…19,25,58 We used the median time from AIDS development to death of 9.2 months (1/μ 3,j = 0.8 years) for the duration of AIDS. 58 These disease progression times were based on studies from a cohort in rural Uganda before the availability of ARV therapy in this area.…”

“…[9][10][11][12][13][14] Models have shown that even a partially effective HIV vaccine with minimal efficacy could have an enormous population impact on the HIV epidemic in high-risk groups or areas with a high prevalence of HIV, such as sub-Saharan Africa, [15][16][17][18][19][20][21][22] but that the magnitude of this impact depends on a fine balance between the efficacy of the vaccine, the vaccine coverage level, and the change in risky behavior. 18,20,[23][24][25][26][27] Although decreases in individual risk behavior would augment the population benefits of such vaccination programs in terms of HIV infections prevented, increases in risk behavior might negate those benefits or even worsen the HIV epidemic. Although we are unable to predict what might occur in a future mass vaccination campaign, 7 results from the few completed vaccine trials to date in North America, Europe, and Asia show that risk behavior generally did not increase, [28][29][30][31][32] despite early reports of increased risk behavior in several small trials.…”