Background: Prospective data on the association of HIV transmission and undetectable viral load (UVL) in homosexual male HIV-serodiscordant couples (HM-SDC) are extremely limited. We report the final results from the Opposites Attract cohort study of HM-SDC in Australia, Bangkok and Rio de Janeiro.
Methods: HM-SDC were recruited through clinics. Information on sexual behaviours was collected at each visit from the HIV-negative partner (HNP). HNPs were tested at baseline and follow-up for HIV antibodies/sexually transmitted infections (STIs), and positive partners (HPPs) for HIV viral load/STIs. Phylogenetic analysis of pol and env genes was performed to identify linked HIV transmissions within couples based on genetic distance and monophyletic grouping. Incidence was calculated per couple-year of follow-up (CYFU), and stratified by pre-exposure prophylaxis (PrEP) use and by whether different forms of condomless anal intercourse (CLAI) were reported. UVL was defined as < 200 copies/mL. One-sided upper 95% confidence limits (UCL) were calculated.
Results: By end 2016, 358 HM-SDC were enrolled: 157, 105 and 96 from Australia, Thailand and Brazil respectively. There were 591 CYFU in 343 couples with at least one follow-up visit of whom 57.4% reported anal sex with outside partners during any point in follow-up. At baseline, 79.9% of HPPs were on anti-retroviral therapy (ART) and 77.9% had UVL; STI prevalence was 14.3%/11.7% in HPPs/HNPs respectively. There were 318 CYFU in periods where CLAI was reported with a total of 16,889 acts of CLAI. There were 3 new HIV infections but no linked transmissions. The overall UCL of the transmission rate when CLAI was reported was 1.16/100 CYFU, and it was 1.56/100 CYFU when there was UVL.

 Linked trans-missions (n)Couple-years of follow up (CYFU)CLAI acts (n)Incidence per 100 CYFU (95% CI)
Overall0591.216,8890 (0-0.62)
Any CLAI0318.016,8890 (0-1.16)
Any CLAI, no daily PrEP0241.312,9280 (0-1.53)
Insertive CLAI0210.08,3890 (0-1.76)
Receptive CLAI0132.14,5690 (0-2.79)
UVL (VL <200)0236.212,6380 (0-1.56)
VL >20005.172900 (0-71.4)
STI diagnosed023.21,0070 (0-15.9)
First 6 months ART010.03410 (0-36.9)
[HIV incidence by category of CLAI]

Conclusions: There were no linked HIV transmissions in almost 600 CYFU involving close to 17,000 acts of CLAI in HM-SDC. Our results provide strong support for the hypothesis that undetectable viral load prevents HIV transmission in homosexual men.