The risk of HIV transmission during anal intercourse may be around 18 times greater than during vaginal intercourse, according to the results of a meta-analysis published online ahead of print in the International Journal of Epidemiology.
Moreover, as well as this empirical work the researchers from Imperial College and the London School of Hygiene and Tropical Medicine carried out a modelling exercise to estimate the impact that HIV treatment has on infectiousness during anal intercourse. They estimate that the risk of transmission from a man with suppressed viral load may be reduced by as much as 99.9%.
Anal intercourse drives the HIV epidemic amongst gay and bisexual men. Moreover a substantial proportion of heterosexuals have anal sex but tend to use condoms less frequently than for vaginal sex, and this may contribute to heterosexual epidemics in sub-Saharan Africa and elsewhere.
Rebecca Baggaley and colleagues conducted a systematic review and meta-analysis (an analysis of all the medical research that meets predefined requirements) of the risk of HIV transmission during unprotected anal intercourse. The same authors have already conducted similar reviews of the transmission risk during vaginal sex and oral sex.
Despite the importance of the topic, only 16 studies were judged to be relevant enough to include in the review. While 12 were conducted with gay or bisexual men, others collected data on heterosexuals who frequently had anal intercourse. All studies were from Europe or North America.