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.
PIP: More than 30% of the general adult populations of some East and Central African cities are infected with HIV. Transmission in Africa South of the Sahara, as opposed to transmission in North America and Europe, occurs primarily through heterosexual intercourse. Some have theorized that infection with sexually transmitted diseases (STD) may facilitate HIV transmission. The high prevalence of STDs in many parts of Africa could therefore play a major role in the rapid heterosexual spread of HIV on the continent. The authors investigated the cofactor effect of genital ulcer disease (GUD) on the risk of HIV transmission during a single heterosexual exposure. Observed risk ratios are expected to be much smaller than per-exposure cofactor effects and to decrease as the observation period lengthens. Data were reanalyzed from longitudinal studies of female prostitutes and men in Nairobi to make rough estimates of the cofactor effects of GUD. The calculations are, however, subject to several caveats and have wide margins of error. The analysis nonetheless suggests that the data are consistent with GUD increasing the risk of male to female transmission by a factor of 10-50, and of female to male transmission by a factor of 50-300. These estimates indicate that GUD may be responsible for a large proportion of heterosexually acquired HIV infections in sub-Saharan Africa, supporting the potential role of STD control as an effective intervention strategy against HIV.