Contribution of sexually transmitted infections to the sexual transmission of HIV

Purpose of review We review recent evidence about the link between sexually transmitted infections (STI) and HIV transmission and consider implications for control programmes. Recent findings New studies and meta-analyses confirm the association of HIV acquisition and transmission with recent STIs, although there is considerable heterogeneity between organisms and populations. Much of the recent evidence relates to herpes simplex virus type 2 (HSV-2), for which the population-attributable risk percentage (PAR%) for HSV-2 is between 25 and 35 in Africa. Mathematical models show how transmission attributable to STI varies with HIV epidemic phase, and HSV-2 becomes increasingly important as the epidemic matures. HSV-2 suppressive therapy reduces HIV concentrations in plasma and the genital tract in people coinfected with HSV-2, in part due to direct inhibition of HIV reverse transcriptase. Recent trials of HSV-2 suppressive therapy have not shown an impact on the risk of HIV acquisition, nor in controlling transmission from dually infected people to their serodiscordant heterosexual partners. Summary Although there is a plausible link between STI and HIV risk, intervention studies continue to be disappointing. This fact does not disprove a causal link, but mechanisms of action and the design and implementation of interventions need to be better understood.