Myron Cohen, MD, University of North Carolina at Chapel Hill
In December, 2011, Science recognised the findings of the HPTN 052 study1 as the scientific breakthrough of the year.2 This study showed a 96% reduction in sexual transmission of HIV in serodifferent couples (one partner HIV positive, the other HIV negative) when the HIV-positive partner was successfully treated with antiretroviral therapy (ART).1 However, the HPTN 052 study included only a small number of men who have sex with men (MSM), for whom HIV acquisition often includes anal exposure, an efficient route of HIV transmission.3 Furthermore, the couples in the HPTN 052 study were counselled to use condoms, so the observed benefits of ART also reflected the contribution of safer sexual behaviours. Accordingly, other investigators4, 5 have subsequently studied HIV transmission in couples who specifically chose not to use condoms.
In 2016, the results of the PARTNER study were reported,4 a study that had enrolled 1166 serodifferent couples at 75 sites in 14 European countries between 2010 and 2014; 340 gay couples were included. The study reported no HIV transmission from ART-treated gay men with viral suppression to their sexual partners, but the upper confidence limit of calculated risk from unprotected receptive anal intercourse with ejaculation was 2·70 per 100 couple-years of follow-up. The estimate for HIV transmission in gay couples was less precise than that obtained for heterosexual couples because of the lower number of couple-years of follow-up accrued; thus, the investigators launched the PARTNER2 study, the results of which are now reported by Alison Rodger and colleagues5 in The Lancet. From 2014 to 2017, the investigators continued recruitment of an additional 495 gay couples. In the PARTNER2 study, 782 couples provided evaluable results over 1593 couple-years of follow-up, including 76 088 self-reported episodes of condomless anal intercourse. The investigators observed no episodes of HIV transmission from the HIV-positive partners to their HIV-negative sexual partners. They concluded that the risk of HIV transmission from an HIV-infected man receiving ART with a plasma HIV-1 RNA (viral load) of less than 200 copies per mL over the preceding year was, remarkably, zero, with a narrow upper 95% CI limit of 0·23 per 100 couple-years of follow-up.
These important results serve to inspire and challenge us. Timely identification of HIV-infected people and provision of effective treatment lead to near normal health and lifespan 6 and virtual elimination of the risk of HIV transmission.1, 4, 5, 7 Yet maximising the benefits of ART has proven daunting, especially for MSM. It is not always easy for people to get tested for HIV or find access to care; in addition, fear, stigma, homophobia, and other adverse social forces continue to compromise HIV treatment. Furthermore, diagnosis of HIV infection is difficult in the early stages of infection when transmission is very efficient,8 and this limitation also compromises the treatment as prevention strategy.9