The contribution of steady and casual partnerships to the incidence of HIV infection among homosexual men in Amsterdam :

The probability of HIV-1 transmission per coital act in representative African populations is unknown. We aimed to calculate this probability overall, and to estimate how it is affected by various factors thought to influence infectivity. 174 monogamous couples, in which one partner was HIV-1 positive, were retrospectively identified from a population cohort in Rakai, Uganda. Frequency of intercourse and reliability of reporting within couples was assessed prospectively. HIV-1 seroconversion was determined in the uninfected partners, and HIV-1 viral load was measured in the infected partners. Adjusted rate ratios of transmission per coital act were estimated by Poisson regression. Probabilities of transmission per act were estimated by log-log binomial regression for quartiles of age and HIV-1 viral load, and for symptoms or diagnoses of sexually transmitted diseases (STDs) in the HIV-1-infected partners. The mean frequency of intercourse was 8.9 per month, which declined with age and HIV-1 viral load. Members of couples reported similar frequencies of intercourse. The overall unadjusted probability of HIV-1 transmission per coital act was 0.0011 (95% CI 0.0008-0.0015). Transmission probabilities 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 probabilities per act did not differ significantly by HIV-1 subtypes A and D, sex, STDs, or symptoms of discharge or dysuria in the HIV-1-positive partner. Higher viral load and genital ulceration are the main determinants of HIV-1 transmission per coital act in this Ugandan population. Show more

Worldwide, the predominant mode of human immunodeficiency virus (HIV) transmission is heterosexual intercourse, but the risk of heterosexual transmission and the effectiveness of measures to prevent it are not well defined. We conducted a prospective study of HIV-negative subjects whose only risk of HIV infection was a stable heterosexual relationship with an HIV-infected partner. Every six months the subjects were interviewed, tested for HIV, and counseled about safe sexual practices. A total of 304 HIV-negative subjects (196 women and 108 men) were followed for an average of 20 months. During the study, 130 couples (42.8 percent) ended their sexual relationships, most often because of the HIV-infected partner's illness or death. Of the 256 couples who continued to have sexual relations for more than three months after enrollment in the study, only 124 (48.4 percent) used condoms consistently for vaginal and anal intercourse. Among these couples, none of the seronegative partners became infected with HIV, despite a total of about 15,000 episodes of intercourse. Among the 121 couples who used condoms inconsistently, the rate of seroconversion was 4.8 per 100 person-years (95 percent confidence interval, 2.5 to 8.4). Eleven couples refused to answer questions about condom use. The risk of transmission increased with advanced stages of HIV infection in the index partners (P < 0.02) and with genital infection in the HIV-negative partners (P < 0.04). Withdrawal to avoid ejaculation in the vagina had a protective effect in uninfected women (P < 0.02). Consistent use of condoms for heterosexual intercourse is highly effective in preventing the transmission of HIV. Among couples not using condoms regularly, the risk of HIV transmission varies widely. Show more