The first, published in 1999, calculated the risk to be 0.06% (equivalent to one transmission per 1,667 exposures). No meta-analysis estimates currently exist for insertive anal sex (inserting the penis into the anus, also known as topping) but two individual studies were conducted to calculate this risk.
(This means that an average of one transmission occurred for every 71 exposures.) This risk was similar regardless of whether the receptive partner was a man or woman. 1 The analysis, based on the results of four studies, estimated the risk through receptive anal sex (receiving the penis into the anus, also known as bottoming) to be 1.4%.
Anal SexĪ meta-analysis exploring the risk of HIV transmission through unprotected anal sex was published in 2010. For types of sex where meta-analysis estimates do not exist, numbers from individual studies are provided. Below are estimates from meta-analyses that have combined the results of studies conducted in high-income countries. The results of several meta-analyses suggest that some types of sex carry on average a higher risk of HIV transmission than others. To come up with a more accurate estimate for each type of unprotected sex, some researchers have combined the results of individual studies into what is known as a meta-analysis. Due to the difficulties of calculating this risk, these studies have produced a wide range of numbers. Several studies have aimed to estimate the average risk of HIV transmission from a specific type of unprotected sex (for example, vaginal/anal/oral insertive/receptive). Researchers can then conclude that any unprotected sex reported by a study participant counts as an exposure to HIV. For this reason, researchers usually enroll HIV-negative individuals who are in stable relationships with an HIV-positive partner (also known as serodiscordant couples). Because a person may have trouble remembering their sexual behaviour or may not want to tell the whole truth, this reporting is often inaccurate.įurthermore, a person does not always know the HIV status of their partner(s). Researchers ask HIV-negative individuals enrolled in these studies to report how many times they have had sex in a given period of time, what type of sex they had, how often they used condoms and the HIV status of their partner(s). To do this effectively, a group of HIV-negative individuals need to be followed over time and their exposures to HIV - both the number of times they are exposed and the types of exposure - need to be tracked.Īs you can imagine, accurately tracking the number of times a person is exposed to HIV is very difficult. It isn't easy for researchers to calculate the risk of transmission from an exposure to HIV through sex.