Receptive unprotected anal intercourse carries ten
times the risk of insertive unprotected anal intercourse, according to a US
study of new infections amongst gay men. However, 15% of those infected reported
just one or two episodes of UAI during the two year study.
The per-contact risk of acquiring HIV infection
during unprotected receptive anal intercourse with a man known to be
HIV-positive was 0.82% (eight in one thousand), whilst the per contact risk of
acquiring HIV from unprotected insertive anal intercourse with any partner was
more than ten times lower (0.6 in one thousand). Receptive oral sex that
involved ejaculation carried a per-contact risk of 0.4 in one
thousand.
Sixty seroconversions were reported amongst 2,189 gay
men recruited to the study between 1992 and 1994 in three American cities. Nine
seroconversions were reported to have occurred after just one or two episodes of
UAI, suggesting that the per contact risk is highly unpredictable. Only 45%
reported unprotected receptive anal intercourse during the year preceding
seroconversion - condom failure appears to have accounted for a substantial
proportion of infections.
The authors also note that the per-contact risk of
unprotected receptive anal intercourse is roughly twice as high as that
estimated for needlestick injuries. The authors argue that this level of
per-contact risk justifies the provision of post-exposure prohylaxis to gay and
bisexual men
Reference
Vittinghoff E et al. Per-contact risk of human
immunodeficiency virus transmission between male sexual partners. American
Journal of Epidemiology 150: 306-311, 1999.