The Lancet recently published the results of the PARTNER2 study which confirms that there is effectively zero risk of sexual transmission of HIV for people living with HIV who are on antiretroviral therapy (ART) and who have an undetectable viral load. The findings of this study support the message that undetectable equals untransmittable (U=U) and demonstrate the importance of early testing and treatment for HIV.

The PARTNER study was a prospective observational study that recruited heterosexual and same-sex couples in which one partner was HIV-positive and taking ART and the other was HIV-negative. Eligible couples reported condomless sex and did not use either HIV pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP). The first phase (PARTNER1) lasted until 2014 and the second phase (PARTNER2), or extension, lasted until 2018 and focused exclusively on same-sex couples. Nearly 800 same-sex couples were enrolled in the PARTNER2 study for nearly 1,600 couple-years of follow-up and reported more than 76,000 instances of condomless anal sex. Over the course of the study, not a single phylogenetically linked HIV infection occurred (i.e., no HIV-positive study participant transmitted HIV to their partner). According to the authors, the “findings provide conclusive evidence that the risk of HIV transmission through anal sex when HIV viral load is suppressed is effectively zero.”

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