AIDS 2018 Co-chairs’ Choice: Report on PARTNERS2 Study.
Treatment as prevention (TasP) is a paradigm shift in HIV prevention modalities. For sero-different couples, TasP research has been undertaken to determine the risk of HIV transmission during condomless sex when the index partner is virally suppressed on ART.
The original PARTNERS1 observational study enrolled sero-different heterosexual and gay couples. For gay male sero-different couples, zero cases of HIV transmission were documented. However, the upper 95% confidence limit of the rate for gay men was 0.84 /100 CYFU. PARTNERS2 was a follow-up study aimed at providing a more precise estimates of transmission risk in gay partnerships. This TasP study, was, in essence, to determine if being undetectable is equated with being untransmissable.
PARTNERS2 had 1) very strict inclusion and exclusion criteria; 2) the ability to phylogenetically link transmissions during eligible CYFU; 3) a robust sample size (783 gay couples); 4) multiple sites (conducted at 75 European clinical sites); 5) median 1.6 years of follow-up; and 6) a high degree of risk of HIV transmission. Among serodifferent gay couples who had sex ~77,000 times without condoms with undetectable viral load (<200 copies), there were zero phylogenetically-linked transmissions during ~1600 CYFU.
PARTNERS2 study is a watershed moment in HIV prevention research history. It is simply one of the most important HIV observational prevention studies to date. Many of the strengths of its design are listed above. The one limitation is its generalizability: 89% of the participants were white European males. Nevertheless, it’s hard to argue with zero transmissions. With such robust data confirming the results of other studies, the jury is now in: Undetectable = Untransmissable (U=U)!