Probability of HIV-1 transmission per coital act in monogamous, heterosexual, HIV-1-discordant couples in Rakai, Uganda

Background The probability of HIV-1 transmission per coital act in represen tative African populations is unknown. We aimed to calculate this probabili ty overall, and to estimate how it is affected by various factors thought t o influence infectivity. Methods 174 monogamous couples, in which one partner was HIV-1 positive, we re retrospectively identified from a population cohort in Rakai, Uganda. Fr equency of intercourse and reliability of reporting within couples was asse ssed prospectively, HIV-1 seroconversion was determined in the uninfected p artners, and HIV-1 viral load was measured in the infected partners. Adjust ed rate ratios of transmission per coital act were estimated by Poisson reg ression. Probabilities of transmission per act were estimated by log-log bi nomial regression for quartiles of age and HIV-1 viral load, and for sympto ms or diagnoses of sexually transmitted diseases (STDs) in the HIV-1-infect ed partners. Results The mean frequency of intercourse was 8.9 per month, which declined with age and HIV-1 viral load. Members of couples reported similar frequen cies of intercourse. The overall unadjusted probability of HIV-1 transmissi on per coital act was 0.0011 (95% CI 0.0008-0.0015). Transmission probabili ties increased from 0.0001 per act at viral loads of less than 1700 copies/ mL to 0.0023 per act at 38 500 copies/mL or more (p=0.002), and were 0.0041 with genital ulceration versus 0.0011 without (p=0.02). Transmission proba bilities per act did not differ significantly by HIV-1 subtypes A and D, se x, STDs, or symptoms of discharge or dysuria in the HIV-l-positive partner. Interpretation Higher viral load and genital ulceration are the main determ inants of HIV-1 transmission per coital act in this Ugandan population.