A new analysis of the risk of HIV transmission posed by specific sexual acts found that younger men who have sex with men and MSM of color have a higher risk of the contracting the virus. Publishing their findings in the Journal of Acquired Immune Deficiency Syndromes (JAIDS), researchers pooled data from three longitudinal studies of MSM from the era preceding modern antiretrovirals (ARVs) and the early-ARV era. Covering overlapping time periods that spanned 1998 to 2003, the studies, which all required the men to be HIV negative upon entry and which gathered reports on their sexual behavior, included the HIV Network for Prevention Trials Vaccine Preparedness Study, the EXPLORE behavioral efficacy trial, and the VAX004 vaccine efficacy trial.

The investigators concluded that unprotected receptive anal intercourse (URAI) with an HIV-positive partner was the most risky act, with an estimated 0.73 percent chance of seroconversion for each act. The per-contact risk (PCR) for URAI with a partner of unknown HIV status was 0.49 percent. For protected receptive anal intercourse (PRAI) and unprotected insertive anal intercourse (UIAI) with an HIV-positive partner, the PCR was a respetive 0.08 percent and 0.22 percent. PRAI with a partner of unknown status had a PCR of 0.11 percent.

The scientists speculate that the similarity they saw in the PCRs between the pre-modern ARV and early-modern ARV eras was a consequence of relatively low proportions of MSM with a suppressed viral load during both eras. Today, just 30 percent of HIV-positive MSM have an undetectable viral load and therefore a 96 percent reduced risk of passing on the virus. A vast increase in the current use of ARVs among MSM with the virus would most certainly drive down PCRs among HIV-negative MSM.

The PCRs were elevated among young MSM. The researchers speculated one reason may be that younger men lack the skills of their older counterparts when it comes to negotiating safer sex, or to using risk reduction strategies such as withdrawing before ejaculation.

There was also evidence that Latino MSM had a higher PCR for UIAI with HIV-positive men. And there was inconclusive evidence that black MSM had higher PCRs than white MSM for URAI and UIAI with HIV-positive men. If these data reflect reality, one reason for the disparities may be that MSM of color report that they are more likely to have sex with a man of their same race or ethnicity. Untreated sexually transmitted infections may also contribute to the disproportionate rates of HIV among minority MSM.