An enormous trove of evidence now supports the increasingly solid global consensus that if you maintain an undetectable viral load with antiretroviral treatment, then you have effectively zero chance of transmitting HIV.

The PARTNER2 study, conducted in 14 European countries, enrolled 972 mixed-HIV-status gay male couples between September 2010 and July 2017. Through April 2018, a total of 783 of these couples provided the study with 1,596 cumulative years of follow-up that qualified for the final analysis. Each couple was followed for a median of 1.6 years.

The final analysis looked only at the study follow-up time during which the HIV-positive partners had a viral load below 200, the HIV-negative partners were not on pre- or post-exposure prophylaxis (PrEP or PEP) and the participants reported condomless anal sex within each couple.

Even after a reported 77,000 condomless sex acts, there were zero transmissions between study partners. (Genetic analyses of viruses helped determine whether any transmissions were linked between partners.) Together with the 12,000 condomless sex acts documented between gay partners in the previously reported Opposites Attract study, researchers now have data on 89,000 such acts between male partners.

Anthony S. Fauci, MD, director of the National Institute of Allergy and Infectious Diseases, concludes “that the body of scientific evidence to date has established that there is effectively no risk of sexual transmission of HIV when the partner living with HIV has a durably undetectable viral load, validating the ‘U=U’ [‘Undetectable = Untransmittable’] message of HIV treatment as prevention.”