The impact of microbicides on HIV and STD transmission: model projections

Aims Current evidence suggests that interventions have had some, but not complete success in raising levels of consistent condom use. In regular partnerships in particular, condom use seldom exceeds 20%-30% of sex acts, even after direct intervention. Consequently, microbicides will have an important role as a fall-back method of protection to condoms. The study aimed to develop and use two mathematical epidemiological models to estimate the additional impact of introducing a microbicide to two different forms of HIV prevention; explore how impact will differ by efficacy; assess how a microbicide may affect STD transmission and its associated benefits for HIV transmission; and how impact may differ between settings. Methods Development of two epidemiological models that simulate the patterns of HIV and STD transmission between groups targeted by 'sex work' and school interventions and their sexual partners. The models use established techniques, and include increased infectivity during the initial HIV high viraemia phase, and the extent to which an STD may facilitate HIV transmission. The models can be used to estimate the short-term impact of adding a microbicide of 40% efficacy to: An intervention promoting condoms among sex workers and their clients in a central African country setting. An in-school youth intervention in a Southern African country. In both cases the projected trends in HIV when (1) there was no intervention; (2) an intervention with 50% coverage among the intervention target group; (3) a microbicide of 40% efficacy against STDs, 40% efficacy against HIV is used by 50% of those reached by the intervention in 50% of sex acts in which a condom is not used. Behavioural and intervention specific data were taken from existing studies. Comparisons between the 3 projected trends were used to obtain estimates of sex worker and youth intervention impact with and without a microbicide. Results Below we show the projected trends in cumulative HIV infections averted by an intervention among sex workers and their clients over time, as a result of adding a microbicide to a sex worker intervention, with initial HIV prevalence of 21%. More generally, the findings highlight: 1. Even a relatively low efficacy microbicide is likely to have a substantial impact on HIV and STD, if its consistent use can be achieved. 2. The importance of the impact of a microbicide on STD transmission, with its associated benefits for HIV transmission. FIGUREFigure3. How epidemiological models can be used to provide insights and to estimate intervention impact.