All exposures are not equal
The results of several meta-analyses suggest that some types of sex carry on average a higher risk of HIV transmission than others. Below are estimates from meta-analyses that have combined the results of studies conducted in high-income countries. For types of sex where meta-analysis estimates do not exist, numbers from individual studies are provided.
Anal sex
A meta-analysis exploring the risk of HIV transmission through unprotected anal sex was published in 2010.
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%. (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.
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. The first, published in 1999, calculated the risk to be 0.06% (equivalent to one transmission per 1,667 exposures).
2 3
Vaginal sex
A meta-analysis of 10 studies exploring the risk of transmission through vaginal sex was published in 2009.
4 It estimated the risk of HIV transmission through
receptive vaginal sex (receiving the penis in the vagina) to be 0.08% (equivalent to 1 transmission per 1,250 exposures).
A meta-analysis of three studies exploring the risk from
insertive vaginal sex (inserting the penis into the vagina) was estimated to be 0.04% (
equivalent to 1 transmission per 2,500 exposures).
4
Oral sex
No meta-analysis estimates exist for oral sex (vaginal or penile) because too few good-quality studies have been completed. This is because it is difficult to find people whose only risk of HIV transmission is unprotected oral sex. A review of the studies that are available was published in 2008 and concluded that vaginal and penile oral sex pose a “low but
non-zero transmission probability.”
5
In the three studies aimed at calculating the risk of HIV transmission from one act of oral sex, no transmissions were observed among three different populations—lesbian serodiscordant couples, heterosexual serodiscordant couples and single gay men—who reported unprotected oral sex as their only risk for HIV transmission. However, these studies enrolled only a small number of people and followed them for only a short period of time, which may explain the lack of HIV transmissions and makes it impossible to conclude that the risk from oral sex is zero.