Female-to-Male Infectivity of HIV-1 Among Circumcised and Uncircumcised Kenyan Men

The vast majority of new HIV infections are heterosexually transmitted, particularly in sub-Saharan Africa, where the pandemic has had the greatest impact. While many biological and behavioral factors likely contribute to country-by-country variation in HIV's spread, ecological and large-survey studies suggest that one explanation may be differences in frequency of male circumcision. The lack of male circumcision has been associated with increased HIV acquisition risk in studies. The current study modeled the per-sex act probability of female-to-male HIV-1 transmission for circumcised and uncircumcised men, including for those with multiple partners.

During 1993-1997, researchers enrolled 992 HIV-negative employees of six trucking companies in Mombasa, Kenya. Demographic, employment, and sexual history were recorded and circumcision status was determined. Data on sexual behavior during the previous three months with each of three partner types (wives, casual partners, prostitutes); number of sex acts in which condoms were used for each partner type; STDs (by physical exam) and HIV (by blood test) were collected in quarterly follow-up visits. Risk-reduction counseling and free condoms were also provided. Of participants, 76 percent returned for at least one visit and were similar to those lost to follow-up in terms of demographics, sexual and condom use history, and circumcision rates. The analysis was based on 745 men, of whom 95 (13 percent) were uncircumcised.

Circumcised men were more likely to be older; Muslim; married; report condom use; and to report extramarital sex. Sexual activity with a wife was reported by 573 men (77 percent); with a casual partner by 474 men (64 percent); and with a prostitute by 182 men (24 percent). The monthly median number of sex acts was 4, of which 3.8 were without condoms. Unprotected sex was common with wives (99 percent), casual partners (85 percent) and prostitutes (71 percent). The majority of sex acts (84 percent) were with wives, followed by casual partners (15 percent) and prostitutes (1.5 percent). Sexual behavior did not significantly differ by circumcision status.

Among the 745 men included for analysis, the overall probability of HIV-1 infection per penile-vaginal sex act was 0.0063. Female-to-male infectivity was significantly higher for uncircumcised than for circumcised men (0.0128 vs. 0.0051). "The effect of circumcision was robust in subgroup analyses and across a wide range of HIV-1 prevalence estimates for sex partners," with approximately a 2- to 3-fold greater infectivity for uncircumcised than for circumcised men across all prevalence estimates for sex partners.

"After accounting for sexual behavior, we found that uncircumcised men were at a >2-fold increased risk of acquiring HIV-1 per sex act, compared with circumcised men," the researchers concluded. "Moreover, female-to-male infectivity of HIV-1 in the context of multiple partnerships may be considerably higher than that estimated from studies of HIV-1-serodiscordant couples. These results may explain the rapid spread of the HIV-1 epidemic in settings, found throughout much of Africa, in which multiple partnerships and a lack of male circumcision are common."

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