Objective: To determine the incidence of heterosexual human immunodeficiency virus type 1 disease transmission and the effect of zidovudine therapy on this risk of transmission.
Design: A cohort of 436 monogamous seronegative female sexual partners of human immunodeficiency virus type 1-infected males was followed up for 740 person-years with regular structured interviews and laboratory tests.
Patients: At enrollment of the women, 50% of their infected partners had one or more signs of disease progression (symptoms of acquired immunodeficiency syndrome, p24 antigen positivity, or CD4+ cell counts lower than 0.4 x 10(9)/L) and 15% were treated with zidovudine.
Main outcome measure: Incidence rates of seroconversion were calculated and relative risks were estimated as incidence rate ratios.
Results: Twenty-seven women seroconverted during follow-up, and the incidence of seroconversion was 3.7 per 100 person-years. Seroconversion was about six times more frequent (relative risk, 5.8; 95% confidence interval, 2.2 to 15.3) in couples not using condoms. Men with signs of disease progression transmitted infection to their partners more frequently and were more frequently treated with zidovudine. When the risk of transmission was estimated accounting for disease progression, the rate of transmission in zidovudine-treated men was lower than in untreated men (relative risk, 0.5; 95% confidence interval, 0.1 to 0.9).
Conclusion: Treatment of human immunodeficiency virus type-1 infected men with zidovudine reduces, but does not eliminate, heterosexual transmission of infection. Behavioral counseling that encourages sexual practices with a lower risk of transmission remains the most important method of prevention.