The Centers for Disease Control and Prevention (CDC) has published a revised estimate of condoms’ effectiveness in preventing HIV acquisition through anal sex, saying that latex blocks nine out of 10 infections, aidsmap reports. In 2015, the CDC published a paper claiming that condoms reduced the risk of acquiring HIV through anal sex by only 70 percent.

At the time, POZ published an analysis sharply critical of the CDC’s methodology in that previous paper.

Publishing their findings in the journal AIDS, CDC researchers conducted a meta-analysis of four studies that included men who have sex with men (MSM), including the JUMP-START study of MSM’s readiness to participate in a vaccine study (published in 1996); the HIVNET vaccine preparedness study (2001); the Project Explore study of an HIV-risk-reduction behavioral intervention (2004); and the behavioral substudy of the VAX004 vaccine study.

The CDC researchers selected only studies conducted before 2005 because in papers published more recently, too high a proportion of the HIV-positive partners with whom men reported having anal sex were on antiretrovirals and had a fully suppressed viral load, rendering them effectively uninfectious.

All the studies provided data on the rate of HIV acquisition during their follow-up periods among initially HIV-negative MSM participants who reported receptive anal sex with men they either knew were HIV positive or men they perceived to have the virus. Additionally, the studies had data on whether men reported using condoms “always,” “sometimes” or “never.” All the studies lasted at least 18 months.

The new paper differs from the 2015 CDC paper on condom efficacy in that it reviewed four previous studies as opposed to just two. And crucially, instead of analyzing condoms’ per-sex-act efficacy, the new paper looked at condoms’ efficacy per additional sex partner. Because the rate of HIV transmission between long-term mixed-HIV-status MSM partners drops off over time, with most transmissions occurring during the first year of a partnership, the authors of the new paper wanted to conduct an analysis in which the HIV risk level was constant. If an HIV-negative man has sex with a series of HIV-positive men, the chance that he will encounter a man with a high viral load indeed remains relatively constant compared with a scenario in which he kept having sex with the same HIV-positive man.

Among the men who reported never or sometimes using condoms, for each additional HIV-positive partner with whom they had receptive anal intercourse, their risk of contracting HIV during their study’s follow-up period increased by 83 percent. For the men who reported always using condoms, their risk per additional such partner rose by just 7.3 percent.

Comparing the difference between these two rates, the study authors concluded that when an HIV-negative man has receptive intercourse with an HIV-positive man, condoms reduce his chance of contracting the virus by 91 percent.

To read the aidsmap article, click here.

To read the study abstract, click here.