266   The Probability of HIV-1 Transmission Per Coital Act in Monogamous HIV-Discordant Couples, Rakai, Uganda.

R. H. Gray*¹, R. Brookmeyer¹, M. J. Wawer², N. K. Sewankambo³, T. C. Quinn¹, D. Serwadda³, and F. Wabwire-Mangen³.
¹Johns Hopkins Univ., Baltimore, MD;²Columbia Univ., New York, NY; and³Makerere Univ., Kampala, Uganda.

Background:The probability of HIV transmission per coital act has not been determined in sub-Saharan Africa.

Methods:174 monogamous HIV-discordant couples were identified retrospectively from a population-based cohort in Rakai, Uganda. Usual frequency of intercourse per month was ascertained, and HIV viral load in HIV- positive partners was determined by PCR. The reliability of frequency of intercourse reported by partners within couples was assessed by paired t tests. Conditional probabilities of transmission were estimated by log-log binomial regression and adjusted to the median viral load (12,476 copies/mL) and age of the population (20—29 years).

Results:Frequency of intercourse was 8.9/month and declined significantly with age and HIV viral load. Members of couples reported comparable frequencies of intercourse. The probability of transmission per contact was 0.0019, declined significantly with age (0.0016 ages 15—19 to 0.0005 ages 40—59 years, p<0.001), and increased significantly with HIV viral load (0.0001 <3,500 copies/mL and 0.0051 50,000 copies/mL, p < 0001), but did not differ between HIV-1 subtypes A and D. Probability of transmission for HIV⁺females was 0.0022 and for HIV⁺males was 0.0013 (p = 0.34). STD diagnoses or symptoms did not significantly affect the risk, with the exception of HIV-negative subjects with positive HSV-2 (0.0041) versus HSV-2 negative serology (0.0005, p = 0.004). Acquisition per act was lower in HIV-negative circumcised men.

Conclusion:The probability of HIV transmission per sex act in Uganda is comparable to that in other populations, suggesting that infectivity of HIV subtypes cannot explain the explosive epidemic in Africa. Viral load and age are the main determinants of the probability of transmission per coital act.