Per-contact risk of human immunodeficiency virus transmission between malesexual partners

The risk of human immunodeficiency virus (HIV) transmission from various ty pes of homosexual contact, including oral sex, is of biologic, epidemiologi c, and public health importance. The per-contact risk of acquiring HIV infe ction from specific acts was estimated in a prospective cohort study of 2,1 89 high-risk homosexual and bisexual men, conducted in San Francisco, Calif ornia; Denver, Colorado; and Chicago, Illinois, in 1992-1994. During 2,633 person-years of follow-up, 60 seroconversions were observed. The estimated per-contact risk of acquiring HIV from unprotected receptive anal intercour se (URA) was 0.82 percent (95% confidence interval: 0.24, 2.76 percent) whe n the partner was known to be HIV+ and 0.27 percent (95% confidence interva l: 0.06, 0.49 percent) when partners of unknown serostatus were included. T here was heterogeneity in per-contact risk, with nine seroconversions occur ring after only one or two episodes of URA. The per-contact risk associated with unprotected insertive anal and receptive oral sex with HIV-positive o r unknown serostatus partners was 0.06 and 0.04 percent, respectively. URA accounted for only 15 percent of all reported sexual activity by seroconver ters. As lower-risk practices become more common, they may play a larger ro le in propagating the epidemic and should also be addressed by intervention s targeting high-risk homosexual and bisexual men.