The potential impact of a moderately effective HIV vaccine with rapidly waning protection in South Africa and Thailand.

BACKGROUND

Although published data from the recent ALVAC/AIDSVAX trial in Thailand (RV144) indicated the HIV vaccine provided very modest protection overall (31.2%), new analysis of trial data has suggested higher efficacy levels earlier in the follow-up period. CDC and UNAIDS organized several modeling research teams to explore the implications of the trial results and potential utility of this vaccine.

METHODS

We explored the impact of a vaccine with moderate but rapidly waning protection (78%, 1.43 years) using an exponential decay function fit to trial data. We varied program coverage levels (20-80%), vaccine efficacy (30-90%), timing (single or multi-year programs), targeting (general or populations at higher risk), and background levels of all other prevention programs (constant or scaled-up). We simulated these various vaccination scenarios in two representative countries using demographic projections generated with Spectrum modeling software. We assumed the vaccine becomes available in 2020 and target coverage is achieved by 2025.

RESULTS

A general vaccination strategy in South Africa covering 60% of the population, for example, would prevent 3.0 million infections between 2020 and 2030-36% of expected infections-and would be very effective, requiring only 39 vaccinations/infection averted. The same strategy in Thailand would prevent 81,000 infections-35% of expected infections-but would require 1725 vaccinations/infection averted. Targeting only populations at higher risk of exposure in Thailand would reduce total vaccinations given by more than ten-fold and would still prevent 52,000 infections-23% of expected infections-while requiring only 220 vaccinations/infection averted. Outcomes were sensitive to program coverage, vaccine efficacy and background levels of all other prevention programs.

CONCLUSIONS

A vaccine with rapidly waning protection could have a substantial impact on the epidemic in South Africa and Thailand. Due to the short duration of effect, large numbers of vaccinations would be needed to maintain high population coverage levels. Further research into the immunological effects of booster vaccinations is warranted.