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New HIV Risk Estimates Are In: Anal Versus Vaginal Sex

By Warren Tong

July 8, 2010

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.

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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.

See Also
Laying It Bare: Gay Men and Unprotected Sex in the Age of HIV/AIDS
More News and Research on Prevention for the HIV Positive
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Reader Comments:

Comment by: anal hygiene (chicago) Wed., Apr. 3, 2013 at 1:26 pm EDT
vaginal sex is better than anal sex because it is more pleasurable to womans in this type of play
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Comment by: fogcityjohn (San Francisco, CA) Wed., Jul. 14, 2010 at 5:53 pm EDT
@ Drew in Atlanta, GA: If you're following the comments, can you please explain what you meant by this sentence in your comment:

"Let's not forget that there are respected scientists who publish in peer-reviewed journals that believe that, given an effective treatment regiment, it IS non-existent."

I'm not aware of anyone who believes that the risk of HIV transmission is non-existent even with effective treatment, much less of any peer-reviewed studies that would support that view. Can you please elaborate?
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Comment by: tugurun Sat., Jul. 10, 2010 at 5:31 am EDT
just forget about numbers, hiv ia a serious illness and no one should take a chance base on statistics, i think is time to to make it pure to the young ones that there should always use condom each an every time because some young adult relay on statistics and found their self caught in middle of the deli ma, what should be done here is to find out why is it so.
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Comment by: Myles Helfand (TheBody.com) Fri., Jul. 9, 2010 at 12:49 pm EDT
@Paul: You are absolutely right from a statistical standpoint, but I don't want us to lose sight of the more important issue here, which both Jeff and Drew referred to: This study may be about numbers, but meta-analysis or no, it's just one study, and the results should not be slotted into some calculation that a person makes about whether to use a condom.

In the same way that I don't recommend anybody look at a study about the average life span of a person diagnosed with HIV and start scheduling their funeral for August 15, 2049, I don't recommend anybody base a safer-sex decision purely on whether there's a one-in-1667 chance HIV will be transmitted the next time they have unprotected anal sex or there's a one-in-74 chance. As some pretty awesome bloggers and community leaders have recently pointed out on our site, the decision to use or not use condoms is complex and multifaceted. This study should be added to that conversation. It shouldn't end it, regardless of the statistical significance of certain findings.
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Comment by: Paul Howard (London) Fri., Jul. 9, 2010 at 6:17 am EDT
I think you have overlooked the decimal point in these statistics. In absolute terms 1.4% and a reduction to 0.06% are both small numbers, but relative to each other as a measure of risk they are very different, this is a reduction in risk exposure of 23 x, not an insignificant fact. The statistical risk is therefore reduced from 1.4 times per 100 individual exposures to 1 per 1666 exposures, statistically this is a large difference.
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Comment by: Jeff (Orlando FL) Thu., Jul. 8, 2010 at 5:42 pm EDT
As someone who ended up in the "other 1 or 2 percent" after very few exposures, I've always had a hard time believing these rosy statistics about the probability of hiv transmission. Suffice it to say, whether you're a top or a bottom, it only takes one unlucky roll of the dice to end up hiv+.
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Comment by: Drew (Atlanta, GA) Thu., Jul. 8, 2010 at 5:15 pm EDT
Thanks for reporting this Warren, it's good to finally have some numbers around these issues. However, your assertion that treatment offers only a "slight reduction" (in italics, no less) is simply incorrect. The most conservative model suggests a reduction of two orders of magnitude, while the other suggests a reduction of three orders of magnitude.

For those unfamiliar with an order of magnitude, think of it this way: three orders of magnitude is the difference between a 6.0 and a 9.0 on the Richter scale. With a magnitude 6.0 earthquake, people might feel a slight tremor, or they might not even notice. With a 9.0 earthquake, an entire city could be devastated, bridges would crack, buildings would topple, etc.

A more fair attack would have been that the study is simply a model, and not based on actual data. And, anyone in science can tell you that models are often grievously inaccurate. But such a criticism would also call into question your assertion that the risk is not (again, italicized) non-existent. Let's not forget that there are respected scientists who publish in peer-reviewed journals that believe that, given an effective treatment regiment, it IS non-existent.

Is the model correct? We can't say. What we can say is the truth about what their model shows: The truth is, three orders of magnitude is an enormous reduction in risk. Two, though smaller, is still quite a large reduction as well. Either way, it is far from "slight."

Am I advocating that everyone run out and have unprotected sex? Not at all. I agree that the level of risk doesn't matter if someone happens to sero-convert. But in our efforts to inform the community, let's analyze the numbers in a fair and scientific way and avoid platitudes that contradict the data being presented.

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