Compared to circumcised men, uncircumcised men are more than twice as likely to acquire HIV each time they have unprotected sex with an infected woman, according to a team of researchers in the US and Kenya. The study the first to measure infectivity, or the probability of HIV-1 transmission per sex act, in a context of multiple partnerships also found that infectivity among men, whether circumcised or not, who have several female partners is many fold higher than estimates based on monogamous HIV-1 discordant couples

Jared Baeten of the University of Washington in Seattle and his colleagues calculated infectivity from data collected during a 4-year prospective study of 745 male employees of six trucking companies in Mombasa, Kenya. Ninety-five of the men were uncircumcised. In quarterly check-ups at a mobile research clinic that visited the companies on a weekly basis, each participant reported his sexual behavior with wives, casual partners and prostitutes during the previous 3 months.

During the study, 11 uncircumcised men and 32 circumcised men acquired HIV. The majority of sex acts reported by the men were with wives—84% versus 15% with casual partners and 1.5% with prostitutes. On average, the men had sex 4.0 times a month, and 3.8 of the encounters did not involve condom use. No participants reported sexual activity with men.

The researchers calculated significantly higher infectivity among the uncircumcised men even when ethnicity, religion, occupation, marital status and other criteria were considered. In general, an uncircumcised man had a 1-in-78 chance of becoming infected during heterosexual sex compared to a 1-in-196 chance for a circumcised male.

The overall probability of female-to-male HIV-1 transmission for a single act of sexual intercourse was 1– in –159, based on all 745 men in the study. The figure is several times greater than infectivity estimates for serodiscordant couples in the US or Europe, which range from 1-in-10 000 to 1-in-1111. However, it is much lower than the odds, ranging from 1-in-33 to 1-in-12, of becoming infected due to a sexual encounter with prostitutes.