Background: Combination antiretroviral therapy (ART) decreases the risk of sexual HIV-1 transmission by suppressing HIV-1 RNA concentrations in blood and genital secretions to below undetectable levels. However, HIV-1 transmission may still occur prior to complete viral suppression.
Methods: Using data from a prospective study of heterosexual HIV-1 serodiscordant African couples (Partners PrEP Study, placebo arm), we quantified HIV-1 transmission risk in 3 time periods: between ART eligibility and treatment initiation, during the first 6 months of ART, and after more than 6 months of ART by when viral suppression is usually achieved. Sexual behavior, self-report of ART use by infected partners, and HIV-1 status of uninfected partners were assessed every 1-3 months. HIV-1 testing was performed using paired rapid antibody tests, with positive results confirmed by ELISA. The primary outcome was phylogenetically-linked HIV-1 transmission within the couple.
Results: We followed 496 uninfected members of serodiscordant couples for 510 person-years. The estimated proportion of unprotected sex acts was 8.1% between ART eligibility and ART initiation, 9.9% during the first 6 months of ART, and 10.8% during > 6 months of ART. HIV-1 incidence in couples eligible but not yet initiating ART was 1.71 per 100 person-years (95% CI: 0.35-5.01, 3 infections in 175 person-years). During the first 6 months after ART initiation, HIV-1 incidence was similar – 1.79 per 100 person-years (95% CI: 0.37-5.22, 3 infections in 168 person-years). There were no transmissions in 167 person-years after >6 months of ART (incidence rate, 0.00 per 100 person-years; 95% CI: 0.00, 2.20).
Conclusions: There was residual risk of HIV-1 transmission during the first 6 months after starting ART, as well as in couples who were eligible for ART but had not yet started treatment. For HIV-1 serodiscordant couples in which the infected partner is starting ART, or is eligible for ART but delays or declines therapy, other prevention options, such as antiretroviral pre-exposure prophylaxis, are needed, in addition to counseling to encourage ART initiation.