The impact of microbicides and changes in condom usage on HIV prevalence in men and women.

The impact on HIV prevalence in men and women of the introduction of vaginal microbicides, and subsequent changes in condom usage, depends critically on the efficacy of microbicides in both blocking transmission to women and reducing the infectiousness of infected women. Neglecting to consider how differences in microbicide efficacy by direction of transmission can affect HIV prevalence over time will result in underestimation of the effect of condom replacement.