Published January, 1999

Per-Contact Risk of Human Immunodeficiency Virus Transmission Between Male Sexual Partners, Eric Vittinghoff et al., American Journal of Epidemiology (1999)

This study of 2,169 homosexual and bisexual men sought to find the rates of seroconversion (becoming HIV positive) during various sexual acts—both protected and unprotected—per sexual contact. Out of the entire group, 60 seroconversions occurred over the two year period between 1992-1994. While the study affirms, unsurprisingly, that unprotected receptive anal sex with a knowingly HIV positive partner carries the highest risk of per-contact infection, the same sex acts with a partner of an unknown serostatus carry a per-contact risk similar to that of needle-stick injuries, and occurred in this study about one-third to one-half as frequently as unprotected receptive anal sex. Also not surprisingly, protection, such as condoms, significantly lowers the risk of infection in both cases.

Other sex acts, such as unprotected receptive oral sex and unprotected insertive anal sex carry an even lower, though still existent, HIV seroconversion risk. The per-contact risk associated with
unprotected insertive anal and receptive oral sex with HIV-positive or unknown serostatus partners was 0.06 and 0.04 percent, respectively.

The authors note, however, that individual risk varies on the basis of factors such as viral load, and that about 15% of infections occurred after only one or two sexual contacts.

Individuals who engaged in unprotected receptive anal intercourse with a partner who was knowingly HIV-positive -- and who consequently were at the highest per contact risk of HIV transmission -- still seroconverted at a rate of only 0.82% per contact, or less than once in 100 acts of unprotected, receptive anal sex.

The authors maintain that the fact of transmission between partners in situations where HIV serostatus is unknown suggests that interventions like post-exposure prophylaxis should not be withheld from individuals who engage in unprotected receptive anal intercourse with a partner of an unknown serostatus, since the risk of transmission is similar to that following needlestick accident involving a HIV-positive individual, for which current guidelines recommend PEP.