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New Study Quantifies HIV Infection Risk

By L.K. Regan

The aggressive use of retroviral therapies has been able to bring many HIV+ men's viral load down to undetectable levels. But until now, it has been unclear whether those medications reduce the risk of infection for those men's sexual partners. Now, a recent study from Australia specifically quantifies how likely a man is to contract HIV from a given sex act—and demonstrates that those risks are barely impacted by contemporary treatment methods.

The study was conducted by researchers at the National Center in HIV Epidemiology and Clinical Research at the University of New South Wales in Darlinghurst, Australia. The researchers studied 1,427 men between 2001 and 2004 who reported having sex with another man at least once in the previous five years. Every six months the subjects were interviewed about their sexual activity and until 2007 tested for HIV. Researchers were interested in relating their results to findings of an American study done in the 1990s, which found an 0.82 percent risk of contracting HIV for every instance of receptive anal sex. But in a result that the study's authors describe as "unexpected," the new study found a slightly higher rate of infection—1.43 percent per instance of receptive anal sex—meaning that the theory that reduced viral loads would make the virus less transmissible appears to be wrong.

That is a disappointing outcome. But within the study's results lies some interesting information quantifying the risk involved in a given sex act. It looks like this: The risk of becoming infected on any single occasion of receptive anal sex with internal ejaculation is 1.43 percent. If the partner withdraws before ejaculating, that risk goes down to 0.65 percent. The infection rate for each occasion of insertive anal sex (topping) is 0.11 percent if a man is circumcised; it goes up to 0.62 percent if a man is uncircumcised. The upshot: receptive anal sex with ejaculation is roughly twice as risky as receptive anal sex with pre-ejaculation withdrawal or topping by an uncircumcised man, and more than 10 times as risky as topping by a circumcised man.

Why should gay men look closely at these numbers? The study's authors warn that "caution should be exercised before interpreting the results at the level of individual men." But blogger and journalist Chris Crain, blogging at Citizen Crain puts a different spin on this study: "I tried for years when I edited the Washington Blade and a number of other gay publications to get this kind of straightforward info about the risk associated with various sexual acts," he writes. "We were, for the most part, stonewalled by public health 'experts' who were loathe to trust gay men with actual info, favoring instead the tired 'AIDS panic' approach of trying to scare men into using condoms by implying that any sort of sex was equally risky." In the end, more accurate information is always an improvement.