Early evidence has suggested a strong association between HIV viral load and the risk of transmission to an HIV-negative partner, as well as a significant reduction in transmission rates with early initiation of antiretroviral therapy and viral suppression among HIV-positive partners. Previous studies have largely examined HIV transmission rates with sexual intercourse in the setting of virally-suppressive therapy in heterosexual couples with variable degrees of condom use. However, evidence around HIV transmission risk through condomless sex in serodifferent gay couples involving an HIV-positive partner on antiretroviral therapy is limited. The previously conducted PARTNER1 study showed no linked transmissions in 888 serodifferent heterosexual and gay couples who endorsed condomless sex while on virally suppressive antiretroviral therapy, but did not provide sufficient follow-up to exclude a significant upper limit of risk for zero transmissions in gay men. In this prospective multicenter observational extension study (PARTNER2) across 14 countries, 972 serodifferent gay couples who reported engaging in condomless sex were recruited between September 2010 and July 2017 and followed up for a median of 2.0 years (IQR 1.1 to 3.5 years) to evaluate incidence rates of HIV transmission. At study visits, participants completed sexual behavior questionnaires, HIV testing for the HIV-negative partner, and viral load testing for the HIV-positive partner. While condomless anal sex was reported a total of 76,088 times in 1593 eligible couple-years, no linked within-couple HIV transmissions were reported (HIV transmission rate of 0%; upper 95% CI 0.23 per 100 couple-years). Of note, 288 (37%) of 777 HIV-negative men in the study reported condomless sex with other partners, and 15 new HIV infections were noted during follow-up, of which all were deemed to be phylogenetically unrelated to the HIV positive partner’s viral haplotype. This study therefore provides conclusive evidence that the risk of HIV transmission in gay couples through condomless anal sex is effectively 0% with viral load suppression. This is in line with previous evidence in heterosexual couples, supporting an “undetectable equals untransmissible” message, and encouraging early testing and initiation of early treatment.
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