March 4, 2010
Although highly active antiretroviral treatment (HAART) can lower HIV in the blood to undetectable levels, it does not reduce the risk of contracting the virus during a single act of unprotected anal sex, according to a new study of men who have sex with men.
Dr. Fengyi Jin, of the National Center in HIV Epidemiology and Clinical Research at the University of New South Wales, and colleagues estimated the HIV risk for each episode of unprotected insertive or receptive anal sex. A total of 1,427 men who reported homosexual sex at least once in the past five years were recruited from June 2001 to December 2004. Participants were interviewed about their sexual behavior every six months and tested for HIV annually until June 2007.
During follow-up, 53 men contracted HIV. The riskiest type of sexual activity was receptive anal sex with ejaculation into the rectum, with a per-contact risk of HIV transmission of 1.43 percent. Withdrawal prior to ejaculation lowered the receptive partner's risk to 0.65 percent. The estimated transmission risk for insertive unprotected sex in participants who were circumcised was 0.11 percent and 0.62 percent for uncircumcised men.
The team noted the findings were "very similar" to those of a US study from the early 1990s: It found a 0.82 percent risk for every instance of receptive anal sex regardless of whether or not withdrawal occurred.
About 70 percent of HIV-infected men are on HAART in Australia, and about three-quarters of these individuals have no detectable virus in their blood. Thus, the authors said it is "surprising" that the risks associated with unprotected sex are similar to what they were in the pre-HAART era.
The team said the findings apply to the population of gay men as a whole, but "caution should be exercised before interpreting the results at the level of individual men." The researchers noted that 12 participants contracted HIV after having unprotected anal sex less than 10 times, while six did not contract the virus even though they had "extremely large numbers" of receptive anal sex episodes with HIV-infected partners.
The study, "Per-Contact Probability of HIV Transmission in Homosexual Men in Sydney in the Era of HAART," was published in the early online edition of AIDS (2010;doi:10.1097/QAD.0b013e3283372d90).