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The first study to examine the probability of HIV infection per act of heterosexual sex among a population with multiple sexual partners has found that uncircumcised men have more than twice the risk of acquiring HIV than do circumcised men.
In the study, published in the Feb. 15 issue of The Journal of Infectious Diseases, Jared Baeten and colleagues from the United States and Kenya collected detailed sexual data from a group of male Kenyan truckers and, using statistical models, developed infectivity measures that estimate the per-sexual-act probability of HIV transmission. The study is the first to calculate the probability of infection for men who have multiple, concurrent heterosexual partners, which was found to be significantly higher than infectivity rates calculated in the past from studies of monogamous couples. Their results may help explain the rapid spread of HIV in settings where circumcision is not common and multiple sexual partnerships are.
Between 1993 and 1997, 745 male employees of trucking companies based in Mombasa, Kenya were followed for the study. Initially they were evaluated for circumcision status and HIV-negativity. Over the length of the study the men were asked to give information concerning the number of sexual encounters with three different partner types--wives, casual partners, and prostitutes--and were screened for HIV and other sexually transmitted infections. At the end of the study the probability of infection was calculated using a statistical model that incorporated published data to estimate the rates of HIV infection among the three types of sexual partners.
For the men in the study, the overall probably of becoming HIV-infected following a single act of intercourse was calculated to be .0063, or one in 160. Uncircumcised men had a more than two-fold increased risk of infection per sexual act compared with circumcised men--one in 80 versus one in 200. Past studies have also found greater HIV infection risk for uncircumcised men. Unlike those studies, however, the present study was also able to take into account cultural characteristics that might be responsible for differences in sexual behavior. These differences could, in turn, account for differences in the risk of infection. But the researchers found that cultural differences in sexual behavior did not matter: when groups of men were excluded from the analysis based on ethnic or religious characteristics, the difference in probability of infection related to circumcision status did not change.
In addition to the results on circumcision, another significant result of the study was the high overall rate of per-contact infection. In past studies that attempted to calculate the probability of female-to-male infection through heterosexual sex, the cohorts consisted only of monogamous couples in which the female partners were HIV-positive. However, in some areas of Africa where HIV infection rates are highest, multiple, concurrent partnerships are more common than monogamous couplings. Past studies of
monogamous couples found that the probability of HIV transmission per-act of sexual intercourse was on the order of one in 1000 or less, much lower than the probability of one in 160 found in this study. Taken with the authors' evidence that female-to-male infection rates may be more than two-fold greater for uncircumcised men, this new information may help us understand why the virus is spreading rapidly in parts of Africa, and help define better HIV strategies for prevention in these settings.