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LB1 Stochastic Simulation to Assess Vaccine Impact on HIV Incidence in Rakai, Uganda
R. Gray*1, X. Li1, M. Wawer2, R. Moore1, D. Serwadda3, N. Sewankambo3, F. Wabwire-Mangen3, T. Lutalo4, and T. Quinn1
1Johns Hopkins Univ., Baltimore, MD, USA; 2Columbia Univ., Sch. of Publ. Hlth., New York, NY, USA; 3Makerere Univ., Sch. of Med., Kampala, Uganda; and 4Rakai Project, Entebbe, Uganda
Background: To estimate the impact of an HIV vaccine on HIV incidence and control of the epidemic in Rakai, Uganda, using empirical data on probability of transmission per sex act.
Methods: A stochastic simulation model was constructed to estimate HIV incidence in a rural African population under differing assumptions of vaccine efficacy in HIV( persons and reductions of viral load in HIV+ persons. Model inputs included empirical data from Rakai, Uganda, on probability of HIV transmission per coital act, associated with HIV viral load, age, gender, and sexual behaviors. Simulations were run for populations with the known age/sex and viral distributions of Rakai. Vaccine assumptions included preventive efficacies in HIV( persons from 25 to 75%, with and without ART initiated under DHSS 2001 guidelines. Simulations also examined potential effects of vaccines in HIV+ recipients, assuming a 0.5 and 1.0 reduction in log10 viral load. We also estimated the incidence required to reduce the reproductive number (Ro) below 1.0, at which point the epidemic would not be self-sustaining.
Results: The model generated HIV incidence (1.4/100 py) approximated the observed incidence (1.5/100 py), suggesting satisfactory fit to empirical data. Simulations suggest a preventive vaccine would require an efficacy of 50% and near complete coverage to reduce transmission below Ro < 1.0, but that this can be achieved by a 75% efficacious vaccine with lower coverage (~50%). Vaccination combined with ART for HIV+ persons can reduce Ro <1.0 even with 25% efficacy if coverage is 75% or more, and with higher vaccine efficacy Ro <1.0 with 50% vaccine coverage. However, a vaccine with even modest reductions of viral load in HIV infected persons (e.g., 0.5 log declines) can reduce Ro below 1.0 with lower population coverage.
Conclusions: Control of the epidemic will require vaccine efficacy >50% and/or ART treatment or a therapeutic vaccine effect on viral load in HIV+ persons.
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