Printed in U.S.A. Role of Viral Load in Heterosexual Transmission of

Follow-up included clinical, behavioral, immunologic, serologic, and virologic evaluations. Two partners were already infected when seen 18 and 34 months after sexual contact began following the infectious transfusion. Four of 17 initially seronegative partners seroconverted during 23 person-years of observation. The recipient's clinical status, mononuclear cell subset variations, and time trend in CD4+ counts had no association with transmission. Individual plasma HIV-1 ribonucleic acid (RNA) loads were stable during observation, and sexual transmission was not attributable to an upward trend or transient burst in viremia. However, recipients who transmitted HIV-1 to their sexual partners had higher mean viral RNA levels than did nontransmitting recipients (4.3 vs. 3.6 log1o copies/ml; p = 0.05). Although this series was small, the prospective observations suggest that viral load was the only characteristic in the recipient that contributed to heterosexual infectiousness. Am J Epidemiol 1997; 146:655-61. blood transfusion; HIV; sexually transmitted diseases; viremia Heterosexual transmission of human immunodefi-ciency virus type 1 (HIV-1) infection has been the subject of many surveys in the United States and Europe because of its long-term potential to enhanc