The introduction of effective drugs against HIV has not changed gay men's risk of contracting the virus during a single act of anal sex, new research from Australia shows.
This finding was "unexpected," the study's authors admit, given that treatment with Aids drugs sharply reduces the concentration of HIV in a person's blood, which would theoretically make it more difficult to transmit the virus. Being the "receptive" partner in unprotected anal sex with an HIV-positive man is gay men's main risk factor for contracting the virus, Dr Fengyi Jin of the National Centre in HIV Epidemiology and Clinical Research at the University of New South Wales in Darlinghurst and colleagues note.
The research however, didn’t extend to assessing risks in heterosexual relationships, where the overwhelming majority of SA cases lie.
Less is known about the risks for the "insertive" partner, they add, although there is evidence that it may be lower for circumcised men.
How the study was done
To estimate the risk of contracting HIV for each instance of unprotected anal sex of either type, the researchers enrolled 1 427 men who reported having had sex with another man at least once in the past five years between June 2001 and December 2004.
The study participants were interviewed about their sexual behavior every six months, and tested for HIV every year 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; each such act carried a 1.43% risk of contracting HIV.
If a man's partner withdrew before ejaculation, the risk dropped to 0.65%.
Circumcised men had a 0.11% risk of contracting HIV for every insertive sex act, while the risk for circumcised men was 0.62%.
Findings similar to 90s study
The findings are "very similar" to a US study done in the early 1990s, the researchers note, which found an 0.82% risk of contracting HIV for every instance of receptive anal sex (whether or not withdrawal occurred).
Rates of HIV testing among gay men in Australia are "very high," the researchers note, while 70% of HIV-positive men are receiving treatment with powerful Aids drugs. And three quarters of these men have no detectable virus in their blood.
So it is "surprising," the researchers say, that the risks associated with unprotected sex are so similar to what they were in the early 1990s, when it would have been rare for an HIV-positive man to have undetectable levels of the virus in his blood.
The findings apply to the population of gay men as a whole, the researchers add, and "caution should be exercised before interpreting the results at the level of individual men."
Genes and other biological factors can influence the risk of both transmitting and contracting the virus, they explain.
For example, they note, 12 of the men in the study contracted HIV after having unprotected anal sex less than 10 times, while there were six study participants who didn't contract the virus even though they had "extremely large numbers" of receptive anal sex episodes with HIV-positive partners. - (Reuters Health, February 2010)