Modeling the population level effects of an HIV-1 vaccine in an era of highly active antiretroviral therapy.

First generation HIV vaccines may have limited ability to prevent infection. Instead, they may delay the onset of AIDS or reduce the infectiousness of vaccinated individuals who become infected. To assess the population level effects of such a vaccine, we formulate a deterministic model for the spread of HIV in a homosexual population in which the use of highly active antiretroviral therapy (HAART) to treat HIV infection is incorporated. The basic reproduction number R(0) is obtained under this model. We then expand the model to include the potential effects of a prophylactic HIV vaccine. The reproduction number R(f) is derived for a population in which a fraction f of susceptible individuals is vaccinated and continues to benefit from vaccination. We define f(*) as the minimum vaccination fraction for which R(f )< or =1 and describe situations in which it equals the critical vaccination fraction necessary to eliminate disease. When R(0) is large or an HIV vaccine is only partially effective, the critical vaccination fraction may exceed one. HIV vaccination, however, may still reduce the prevalence of disease if the reduction in infectiousness is at least as great as the reduction in the rate of disease progression. In particular, a vaccine that reduces infectiousness during acute infection may have an important public health impact especially if coupled with counseling to reduce risky behavior.