The major PARTNER study contributed a trove of data to support the increasing consensus that fully suppressing HIV with treatment is associated with effectively no risk of transmission. None of the HIV-negative members of the couples studied contracted the virus from their HIV-positive primary partner thanks to the latter participant’s undetectable status. However, among the gay couples in the study, the men in the HIV-negative group contracted the virus at a high rate (2.3 percent per year) from men outside of their primary partnership, according to genetic analyses of their viruses.

In addition, this European group of men who have sex with men used pre-exposure prophylaxis (PrEP) and post-exposure prophylaxis (PEP) at a low rate. Among the 35 percent of the HIV-negative participants who reported having condomless anal sex with men outside of their relationships, PEP or PrEP was used for protection only about 12 percent of the time.