Published interim findings from an ongoing study of mixed-HIV-status heterosexual and gay male couples, which saw no transmissions within couples, underline the power of antiretrovirals (ARVs) to curb the spread of the virus. Preliminary findings from the PARTNER study, which includes 1,116 mixed-HIV status couples in which the HIV-positive partner is on suppressive ARV treatment, were first reported at the 2014 Conference on Retroviruses and Opportunistic Infections (CROI) in Boston.

The study was conducted at 75 clinical sites in 14 European countries. The new analysis, published in the Journal of the American Medical Association, examines data from September 2010 to May 2014, focusing on 888 couples, 62 percent of which were heterosexual couples and 38 percent of which were gay male couples. Between them, the couples provided 1,238 cumulative years of follow-up, or a median 1.3 years. The analysis focused on periods when the HIV-positive partner had a viral load below 200 and when the couples reported condomless sex.

When starting the study, the couples reported they had engaged in condomless sex for a median two years. A total of 108 (33 percent) of the HIV-negative men who have sex with men (MSM) reported condomless sex outside their primary partnership, as did 21 (4 percent) of the HIV-negative heterosexuals.

During the study follow-up period, the couples reported a median 37 incidents of condomless sex per year, including a cumulative 22,000 such acts among the gay couples and 36,000 among the heterosexual ones.

Eleven of the HIV-negative partners contracted the virus during the follow-up period, including 10 MSM and one heterosexual individual; eight of this group reported sex outside their partnership. However, genetic analyses showed that none of these transmissions apparently occurred within those primary partnerships. In other words, there were no HIV transmissions between partners when the HIV-positive individual had a viral load below 200.

The researchers cannot at this point rule out that there is a risk of HIV transmission with such a low viral load, particularly for anal sex and when considering how risk may accumulate over a period of several years. Greater follow-up is needed.

“Although these results cannot directly provide an answer to the question of whether it is safe for serodifferent [mixed-HIV status] couples to practice condomless sex,” the study authors write, “this study provides informative data (especially for heterosexuals) for couples to base their personal acceptability of risk on.”

In an accompanying editorial, Eric S. Daar, MD, and Katya Corado, MD, of the Harbor-University of California, Los Angeles, Medical Center, write: “For individuals who want to routinely or intermittently not use condoms with an HIV-infected partner, clinicians can indicate that the risk of HIV transmission appears small in the setting of continued viral suppression, emphasizing that the duration the HIV-infected partner needs to be virologically suppressed before achieving optimal protection is unknown, although appears to be for at least 6 months, based on the best available data.”

They continue: “Moreover, clinicians need to be clear that even though the overall risk for HIV transmission may be small, the risk is not zero and the actual number is not known, especially for higher-risk groups such as MSM. Although more research is needed with larger numbers of couples and longer follow-up, it is not known if or when such data will emerge. Consequently, for now, clinicians and public health officials must share the data that exist in an honest and understandable way so that serodiscordant couples can be fully informed when individualizing their decision making about sexual practices.”

Editor’s note: These findings were subsequently presented at the 21st International AIDS Conference in Durban, South Africa (AIDS 2016).

To read a press release about the study, click here.

To read the study abstract, click here.