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Background:  Vaginal microbicides are chemical compounds that are in development to prevent or reduce the transmission of HIV when applied topically. Vaginal microbicides may provide an alternative mechanism of protection for women whose partners are unwilling to use condoms, and could also be applied in addition to a condom. The following questions are addressed:  Would the introduction of vaginal microbicides substantially reduce the risk of female sex workers’ acquiring HIV? Which factor would it be most important to maximize, microbicide efficacy or microbicide use? What level of microbicide efficacy and use would be necessary to counterbalance a possible reduction in condom use?

Methods:  We developed risk equations and performed Monte Carlo simulations to model a female sex worker’s daily risk of HIV acquisition currently, and after, microbicide introduction. We used uncertainty and sensitivity analyses, and tornado plots for 2 ranges of microbicide efficacy (30% to 50%) and (50% to 80%). Risk was estimated for female sex workers whose clients sometimes (10% to 50%) use condoms, and those whose clients never use condoms. We estimated the break-even point at which reducing condom use increases risk.

Results:  For both groups of female sex workers, daily risk would decrease ~20% or ~30% using 30% to 50% or 50% to 80% effective microbicides, respectively. Increasing microbicide use would have greater effect on reducing risk than increasing microbicide efficacy. Whether reducing condom use will increase (or decrease) risk will depend on microbicide efficacy, level of pre-microbicide condom usage, and microbicide usage.

Conclusions:  Microbicides could substantially reduce female sex workers’ risk of acquiring HIV; absolute decrease in risk would be greatest in high-prevalence regions. Public health effect of microbicides will depend on usage and efficacy. Even if the microbicides that become available are only low to moderately effective, the probability that risk in female sex workers will increase due to replacing condoms with microbicides is low.

Keywords: Vaginal Microbicides; Female Sex Workers; Risk Equations