Risk of male to female transmission of HIV unexpectedly high in South Africa

It is well known that women in sub-Saharan Africa have the highest risk of HIV infection, with a prevalence of 21% among women aged 21 years. Now it seems that the risk of HIV transmission from men to women is much higher than previously reported. A recent study published in AIDS, shows that this prevalence occurs in young women with few lifetime partners, suggesting that the rate of transmission from men to women is higher than that from women to men.

Researchers recruited 11 904 young South African in 2003 and tested them for HIV using saliva tests. All sexually experienced participants were asked about lifetime partners, their first sexual partner and the last 3 sexual contacts in the past 12 months. Of the 3993 women for whom data were available, the mean reported number of partners was 2.3. The overall HIV prevalence among sexually active women was 21%. There was a significant increase in prevalence with increasing numbers of partners, from 15.2% among women reporting one partner to 28.5% among women reporting 3 or more partners. Analysis of the data, adjusted for confounding factors such as failure to report forced sex, suggested a male to female transmission rate of more than 70%. Conservative estimates of the transmission rates per sexual encounter ranged from one in fifty to one in 16. Factors related to high transmission are sexually transmitted infection and acute HIV infection. Young women are more susceptible when pregnant, when they have abnormal vaginal flora and when using hormonal contraception. Other studies in South Africa show similar findings, and the authors call for urgent intervention through prevention programmes targeting young women.

Pettifor AE et al. AIDS 2007; 21: 861 – 865