Background: Although zero cases of HIV transmission in gay men have been reported in observational studies (PARTNER1 and Opposites Attract) of serodifferent couples where the positive person was on suppressive ART, the level of evidence for gay men remained less than for heterosexual couples. The aim of PARTNER2 was to provide more precise estimates of transmission risk through condomless-sex in serodifferent gay male couples where the HIV-positive partner was on suppressive ART.
Methods: The PARTNER Study was a prospective observational study in 14 European countries. Phase 1 (Sep 2010-May 2014) recruited both heterosexual and gay male couples, and Phase 2 (to April 2018) recruited and followed gay couples only. Study data, collected at baseline and every 6 -12 months, included sexual behaviour questionnaires with HIV testing (HIV-negative partner) and HIV-1 viral-load (HIV-positive partner). Eligibility criteria for CYFU inclusion were condomless sex, no reported PEP or PrEP use, and most recent plasma HIV-1 RNA load < 200 copies/mL in the last year. If a seroconversion occurred, anonymised phylogenetic analysis compared HIV-1 polymerase and envelope sequences in both partners to identify linked transmissions.
Results: Between Sep 2010 and July 2017, 972 gay couples were enrolled. Of these, 779 couples provided 1561 eligible CYFU over a median of 1.6 years [IQR 0.8-2.8]. At baseline, mean age was 40 years [IQR 33-46] and couples reported condomless-sex for a median of 1.0 years (IQR 0.4-2.9). During eligible CYFU, a total of 74,567 condomless-sex acts were reported, a median of 42 times per couple year (IQR 19-74). Condomless-sex with other than the main partner was reported by 285 HIV-negative men (37%). There were 17 new HIV infections, but none were phylogenetically linked transmissions, giving a precise rate of within-couple HIV transmission of zero, with a narrow upper 95% confidence limit of 0.24/100 CYFU. This upper-limit for condomless anal sex with ejaculation was 0.59/100 CYFU.
Conclusions: Despite almost 75,000 condomless-sex acts in gay serodifferent couples where the positive partner was on suppressive ART, we found zero cases of within couple HIV transmission. PARTNER2 provides a similar level of confidence for gay men as for heterosexual couples in PARTNER1.

Figure 1: Rate of HIV Transmission According to Sexual Behavior Reported by the HIV-Negative Partner
[Figure 1: Rate of HIV Transmission According to Sexual Behavior Reported by the HIV-Negative Partner]