New HIV Risk Estimates Are In: Anal Versus Vaginal Sex

When it comes to calculating risk of HIV transmission, some people can really get fixated on specific percentages. That's not something to condone. But it's still interesting -- and useful -- when studies use those percentages to shed new light on key topics in HIV transmission. Which is what a group of British researchers did recently for the age-old question: When it comes to HIV/AIDS, how much riskier is anal sex than vaginal sex?

Researchers from Imperial College and the London School of Hygiene and Tropical Medicine did a meta-analysis of 16 studies related to HIV risk during unprotected anal sex. Most of the studies involved gay or bisexual men. (Our friends at NAM recently summarized the findings in detail.) They estimated that HIV transmission risk during a single act of unprotected, receptive anal sex may be 18 times higher than unprotected, receptive vaginal sex: 1.4 percent compared to 0.08 percent. The estimated risk for unprotected, insertive anal sex was, as expected, found to be lower (0.62 percent) than for receptive anal sex -- and lower still if the man is circumcised (0.11 percent). Still, the risk percentages are all higher for anal sex than vaginal sex, which is in line with earlier study findings.

The researchers went on to look at how HIV treatment might reduce transmission risk. Since the majority of the studies they looked at were done during the pre-HAART era, they used two mathematical models to predict the transmission risk in individuals with a suppressed viral load on modern antiretroviral therapy. The first model predicted that, in this case, there was just a 0.06 percent chance of HIV being transmitted from an insertive partner to a receptive partner during a single act of unprotected anal sex -- that's 96 percent less risk than if the insertive partner were not on HIV treatment. The second model found the risk would be 0.0011 percent, which is 99.9 percent lower than without treatment.

It's easy to get confused by or bogged down in all these numbers, but here is the gist: These estimates lend some truth to the notion that, overall, HIV transmission risk is decreased if the HIV-positive partner has an undetectable viral load. However, it's only a slight reduction when you're talking about overall numbers: The risk on a per-act basis is still very similar (1.4 percent off treatment versus .06 percent on treatment). And it is not non-existent.

So each time a person has sex without a condom with an HIV-positive partner, they're still taking a risk -- and the difference between 98 percent safe and 99 percent safe doesn't mean much to a person who ends up in the other 1 or 2 percent.