Viral Load and Risk of Heterosexual Transmission of HIV-1 among Sexual Partners

T. C. QUINN*, M. J. WAWER, N. SEWANKAMBO, D. SERWADDA, C. J. LI, F. WABWIRE-MANGEN, M. MEEHAN, T. LUTALO, and R. H. GRAY‚ NIAID. Johns Hopkins Univ., Baltimore, MD; Columbia Univ., New York, NY; Makerere Univ., Rakai Project Team, Uganda

Objective: To determine the effect of viral load on the risk of heterosexual transmission of HIV, 415 HIV-discordant (HIV-positive/HIV-negative) couples were followed over a 30-month period in a study of 15,127 people in Rakai district, Uganda. Incidence of transmission and acquisition/100 person-years (py) were determined in relation to HIV viral load, sociodemographic and behavioral characteristics, and active STDs.
Results: The male partner was HIV infected in 228 (55%) and the female was HIV positive in 187 (45%). 90 (21.7%) of 415 discordant couples seroconverted (incidence 11.8/100 py). The transmission rate from males to females was 12.0/100 py and not statistically different from female-to-male transmission (11.6/100 py). Incidence was highest among 15 to 19-year-olds (15.2/100 py) and among uncircumcised males (16.7/100 py); no seroconversions occurred in circumcised males (p<0.0001). Mean viral load was significantly higher in transmitters vs. non-transmitters (90,254 vs. 38,029; p=0.01). There was a dose response of higher transmission with increasing viral load (p<0.0001), with no transmissions <1500 copies/mL, and up to 23.0/100 py for a viral load >50,000 copies/ml. In multivariate analyses, HIV viral load was the major predictor of HIV risk; each log₁₀increment in viral load was associated with a 2.45-fold increased risk of transmission (95% CI 1.85–3.26).
Conclusion: HIV-1 viral load was the major predictor of heterosexual transmission, with no differences by gender. However, circumcision among men reduced risks of acquisition. With no transmissions at viral loads <1500 copies/ml, reductions in viral load with antiretrovirals or vaccines may reduce transmission of HIV-1.

Key Words: heterosexual, transmission, viral load