The cofactor effect of genital ulcers on the per-exposure risk of HIV transmission in sub-Saharan Africa.

The goal was to estimate the cofactor effect of genital ulcer disease (GUD) on the risk of HIV transmission during a single heterosexual exposure. The relation between the risk ratio observed in an epidemiological study and the per-exposure cofactor effect was investigated. Given simple assumptions, we show that observed risk ratios are expected to be very much smaller than per-exposure cofactor effects and to decrease as the observation period increases. Data from longitudinal studies of female commercial sex workers and men in Nairobi were reanalysed. The data are consistent with GUD cofactor effects per sexual exposure of 10-50 for male to female transmission, and of 50-300 for female to male transmission. Although subject to wide margins of error, these estimates indicate that GUD may be responsible for a high proportion of heterosexually acquired HIV infections in sub-Saharan Africa, supporting the potential role of STD control as an effective intervention strategy against HIV.