Objectives: HIV-1 heterosexual transmission among individuals on antiretroviral treatment (ART) with undetectable viremia is extremely rare. The aim of this study was to evaluate the risk of sexual HIV-1 transmission and other sexually transmitted infections (STIs) in HIV-1 serodifferent couples while the index partner is on ART. Methods: HIV transmission was evaluated in 200 HIV-1 heterosexual serodifferent couples in a stable relationship (≥3 months). All HIV-positive individuals had been on ART for ≥3 months and had been followed up for a median preceding time of 4.5 years (range 0.3–16 years) at the HIV couples clinic at Hospital Nossa Senhora da Conceição in Porto Alegre, Brazil. Following written informed consent, participants responded to demographic/behavioral questionnaires. Quantitative PCR for HIV RNA, T-cell subsets, and STI testing (syphilis, herpes, human papillomavirus, gonorrhea, and bacterial vaginosis) were performed. Self-collected vaginal swabs were obtained for quantitative HIV genital viral load testing. Results: Among 200 couples, 70% of index partners were female. Five seroconversions were observed; the HIV infection incidence was 2.5% (95% confidence interval 0.8% to 5.7%). Mean plasma viral load results were higher in HIV transmitters compared to non-transmitters (p = 0.02). The presence of STIs was significantly greater in couples who seroconverted (60.0% vs. 13.3%; odds ratio 9.75, 95% confidence interval 1.55–61.2; p = 0.023). The duration of undetectable HIV viremia and presence of STIs were associated with HIV transmission. Conclusions: Undetectable viremia was the main factor associated with non-transmissibility of HIV in this setting
