The prime-boost HIV vaccine regimen used in the recent RV144 trial resulted in modest efficacy of 31% over 3.5 years, but was substantially higher in the first year post-vaccination. We sought to explore the potential impact of a vaccine with rapidly waning efficacy in a South African population.
We explored two strategies using a dynamic compartmental epidemic model for heterosexual transmission of HIV:  vaccination of a single cohort (30%, 60% or 90% of the initial population), with exponentially waning efficacy, but booster vaccinations at 5- or 2-year intervals, and  continuous vaccination of the unvaccinated population at the same coverage levels (30%, 60% or 90%) but with a constant efficacy vaccine of short duration. We also examined potential changes in post-vaccination condom use.
The single cohort vaccination strategies did not have a substantial impact on HIV prevalence, although without boosters they still prevented 2-6% of the expected infections at 20 years, depending on the population coverage. The 5-year and 2-year booster strategies prevented 8-24% and 17-45% of the expected infections, respectively. Continuous vaccination to maintain population coverage levels resulted in more substantial reductions in population HIV prevalence and greater numbers of infections prevented: HIV prevalence at 20 years was reduced from 23% to 8-14% and the number of expected infections was decreased by 34-59%, depending on the population coverage level. Moderate changes in post-vaccination condom use did not substantially affect these outcomes.
An HIV vaccine with partial efficacy and declining protection similar to the RV144 vaccine could prevent a substantial proportion of HIV infections if booster vaccinations were effective and available. Our estimates of the population impact of vaccination would be improved by further understanding of the duration of protection, the effectiveness of booster vaccination, and whether the vaccine efficacy varies between subpopulations at higher and lower risk of exposure.