The RV144 trial is to date the only demonstration of a partially effective HIV vaccine. Preliminary results from HVTN100 suggest similar immunogenicity for an ALVAC HIV clade C vaccine. Efficacy studies of ALVAC clade C with gp120 will include an additional booster at 12 months. Here, the population-level impact and maximum acceptable price of such a vaccine in South Africa are explored using EMOD, an individual-based network model.
The time profile of efficacy of an ALVAC/gp120 regimen with 12-month boost was fit to RV144 results or increased to 50% cumulative efficacy at 24 months. This time-varying efficacy profile was implemented in EMOD-HIV v2.5, a microsimulation model that has been fit to the HIV epidemic, and includes demographics, risk stratification, and HIV testing/cascade of care. In future projections, we varied the scale-up of treatment to examine its interplay with the 20-year impact of a vaccine to be started in 2027. Coverage was assumed to be 50% or 80% with boosters to continue two-yearly for ten years with up to 20% attrition per dose.
A partially effective vaccine could reduce HIV incidence in South Africa by up to 21% with 80% coverage. The most efficient ages to start immunization are 15 in women and 20 in men, with 16-29 HIV infections averted per 1000 individuals vaccinated. If gender differences in vaccination age were impractical to implement, then ages 15 and 18 would be equally optimal for vaccination, with 10-23 HIV infections averted per 1000 vaccinated, a result highly sensitive to concurrent scale-up of ART. In contrast, combining HIV vaccination with the current HPV vaccine program among 9-11 year olds would result in less than one HIV infection averted per 1000 vaccinated due to waning immunity. Maximum cost-effective prices of a vaccine, calculated from the ratio of the net budget impact to DALYs averted, varied widely depending on cost-effectiveness thresholds and ART scale-up, and to a lesser extent on efficacy, age at vaccination, and attrition.
Partially effective HIV vaccines with rapid waning of immunity could substantially reduce HIV incidence if vaccination schedules were aligned with the ages of highest HIV incidence and high coverage levels were achieved. Reaching a new target population with a complex immunization schedule not aligned with other schedules may pose an implementation challenge in South Africa.