How Much HIV Risk Is There For Various Types of Sex?

Human immunodeficiency virus (HIV) digital artwork. Maciej Frolow/Photodisc/Getty Images

One of the questions people most often ask me is what their risk is of getting an STD each time they have sex. My default answer is that there isn't an answer. There are too many factors involved - whether their partner has one or more STDs, whether they are already infected with a disease that could increase their risk, how they have sex - and there also aren't any good ways to do the study. After all, in order to find out how likely infection with a particular STD is during any given sex act, you'd need to have a large pool of people with STDs who are having that sort of sex with uninfected partners, they'd have to eschew protection...

and you'd have to not treat them. It's not exactly an ethical study to do, and though there are ways around the problems, they aren't very efficient.

Still, every once in a while someone tries to ask the question.

Australian scientists recently published a study looking at the risk of acquiring HIV from having anal sex during the era of highly active anti-retroviral therapy (HAART). They recruited over 1000 HIV-negative men and, every 6 months, they asked them about their experiences with unprotected anal sex - including number of partners, partner's HIV status, and whether they had receptive or insertive sex. The study followed these men for between 3-6 years and also repeatedly tested them for HIV. Sexual episodes when a condom slipped or broke were included in the "unprotected" count. Then they did some statistical magic.

Specifically what they did was make two assumptions. The first was that there was a 10% probability of any partner of unknown status actually being HIV positive.

The second was that there was a 0.3% probability that any partner thought to be negative was HIV positive. Using those assumptions they calculated that the risk of getting HIV during any unprotected anal sex encounter was:

  • 0.65% for receptive anal intercourse without ejaculation
  • 1.43% for receptive anal intercourse with ejaculation
  • 0.11% for insertive anal intercourse (circumcised)
  • 0.62% for insertive anal intercourse (uncircumcised)

What do those numbers really say about absolute risk? I hesitate to say, since they're based on assumptions that may or may not be accurate, but I do find the relative risk calculations to be quite interesting. To me, the three take home messages are 1) for uncircumcised men, insertive sex and receptive sex without ejaculation are similarly risky, 2) ejaculation more than doubles the risk of receptive anal sex, and 3) circumcision seems to greatly reduce the risk of infection during insertive sex.

There is one additional point made by the study researchers - which is that their per-contact risk calculation implies that the risk of infection during the HAART era isn't particularly different than that seen in studies done in the era before such treatments were available. This is surprising, but it could be due to any one of a number of factors. The estimates of how many men in the population are infected could be low, most men could become infected by partners who are not yet being treated, or viral load might not be as big of a factor in anal transmission as it is during vaginal transmission.

It's too soon to tell which explanation will prove to be relevant, or if a combination of factors will turn out to be involved. It's a fascinating study though, less because of the results than because of what it shows us we still don't know... and because it clearly demonstrates why there is no simple answer to the question "how much am I really risking each time I have sex?"


Jin F, Jansson J, Law M, Prestage GP, Zablotska I, Imrie JC, Kippax SC, Kaldor JM, Grulich AE, Wilson DP. Per-contact probability of HIV transmission in homosexual men in Sydney in the era of HAART. AIDS. 2010 Mar 27;24(6):907-13. doi: 10.1097/QAD.0b013e3283372d90.

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