U=U in PARTNER2 Study of MSM

PARTNER1, whose results were reported last year,(1) was one of the studies that supported current efforts to promote the growing scientific consensus supporting "Treatment as Prevention" (TasP) or Undetectable = Untransmittable (U=U).

PARTNER1 evaluated HIV transmission risk in a European multicenter observational cohort of HIV serodifferent heterosexual and MSM couples in which the HIV-positive partners were on suppressive ART (HIV RNA <200 copies/mL) and in which condoms were not used. PARTNER1 found zero cases of linked (within-couple) HIV transmission in either the heterosexual or the MSM couples. These results, while very exciting, were interpreted somewhat cautiously in regard to MSM, because there were fewer data for MSM than for heterosexual couples, resulting in a less-precise estimate of HIV transmission risk (specifically, a broader confidence interval [CI]) than for heterosexual couples.

PARTNER2, presented at AIDS 2018, addresses the need for more data on HIV transmission risk for HIV-positive MSM on suppressive ART in serodifferent partnerships. PARTNER2 followed 783 MSM serodifferent couples (89% of the HIV-negative partners were white; median age was 38) who practiced condomless anal sex.(2) At baseline, the partner with HIV had been on ART a median of 4 years, and 98% reported ≥90% adherence to ART. Follow-up data were included only for time intervals in which the couples had condomless sex, there was no use of PEP or PrEP, and the HIV-positive partner's HIV RNA was <200 copies/mL. During the study, 37% of the HIV-negative partners reported condomless sex with partners other than the study partner, and 23% had STIs (27% of the HIV-positive study partners had STIs).

Couples were followed for a median of 1.6 years, accruing 1,596 couple years of follow-up and a total of 76,991 episodes of condomless anal sex. As in PARTNER1, there were no linked HIV infections. New infections did occur in 15 partners who were negative at baseline, but these were not genetically related to the study partner's virus, ie, transmission occurred from partners outside the study partnership.

Statistical analysis yielded an upper limit of the 95% CI of 0.23 per 100 couple years (lower than that for heterosexual couples in PARTNER1 [0.43]), though it was higher if the negative partner had anal receptive sex (0.43 and 0.57 without and with ejaculation, respectively), and in particular if he had an STI (2.74); accrual of more data may yield narrower CIs in the future.

Clinical Bottom Line

PARTNER2, like the PARTNER1 and the Opposites Attract (3) studies, showed zero cases of HIV transmission in serodifferent MSM couples in which the seropositive partner was on suppressive ART. These data in MSM match those from studies of HIV transmission risk in male-female serodifferent couples (eg, PARTNER1 and HPTN 052 [4]). From PARTNER 1, PARTNER2, and Opposites Attract alone we have reports of more than 150,000 condomless sex acts between serodifferent couples, with no cases of HIV transmission from the HIV+ partner on ART. The contribution of the PARTNER2 data should fortify our confidence in supporting U=U for serodifferent sexual partnerships in which the HIV-positive partner maintains continuous HIV suppression on ART.


Rodger AJ, Cambiano V, Bruun T, et al; PARTNER Study Group. Sexual activity without condoms and risk of HIV transmission in serodifferent couples when the HIV-positive partner is using suppressive antiretroviral therapy. JAMA. 2016 Jul 12;316(2):171-81. PMID: 27404185.

Rodger A, Cambiano V, Bruun T, et al; PARTNER Study Group. Risk of HIV transmission through condomless sex in MSM couples with suppressive ART: the PARTNER2 Study extended results in gay men. In: Program and abstracts of the 22nd International AIDS Conference; July 23-27, 2018; Amsterdam. Abstract WEAX0104LB.

Bavinton B, Grinsztejn B, Phanuphak N, et al. HIV treatment prevents HIV transmission in male serodiscordant couples in Australia, Thailand, and Brazil. In: Program and abstracts of the 9th IAS Conference on HIV Science; July 23-26, 2017; Paris. Abstract TUAC0506LB.

Cohen MS, Chen YQ, McCauley M, et al; HPTN 052 Study Team. Antiretroviral therapy for the prevention of HIV-1 transmission. N Engl J Med. 2016 Sep 1;375(9):830-9. PMID: 27424812.

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