New HIV infections among the HIV-negative gay men in the PARTNER study, due to sex with partners outside the main relationship, was high, a recent conference heard.
PARTNER made headlines by demonstrating that there were no transmissions from an HIV-positive partner who was on antiretroviral therapy and virally suppressed in almost 60,000 acts of condomless sex. These data allowed the researchers to establish the maximum possible likelihood of transmission, and to announce that, most likely, the chance of an HIV-positive partner with a fully suppressed viral load of below 200 copies/ml passing on HIV was zero. PARTNER provides crucial evidence for the U=U (Undetectable = Untransmittable) campaign.
However, there were HIV infections in PARTNER: eleven of them by 2016, ten in gay men. In all cases, however, phylogenetic testing showed that the infecting virus came from someone other than the primary partner.
Each year, 2% of HIV-negative gay male partners acquired HIV. Looking only at those men who reported having condomless anal sex with non-primary partners, each year 7% acquired HIV.
In short, men whose main partner is undetectable are not safe from HIV if they are also having condomless sex with other people. In this situation, it would make sense for the HIV-negative man to use post-exposure prophylaxis (PEP) or pre-exposure prophylaxis (PrEP).
But very few of those taking part in the PARTNER study did so, resulting in these high levels of infection.