Maternal viral load and timing of mother-to-child HIV transmission, Bangkok, Thailand

Objectives: To determine the proportion of HIV-1-infected infants infected in utero and intrapartum, the relationship between transmission risk factor s and time of transmission, and the population-attributable fractions for m aternal viral load. Design: Prospective cohort study of 218 formula-fed infants of HIV-1-infect ed untreated mothers with known infection outcome and a birth HIV-1-positiv e DNA PCR test result. Methods: Transmission in utero was presumed to have occurred if the birth s ample (within 72 h of birth) was HIV-1-positive by PCR; intrapartum transmi ssion was presumed if the birth sample tested negative and a later sample w as HIV-1-positive. Two comparisons were carried out for selected risk facto rs for mother-to-child transmission: infants infected in utero versus all i nfants with a HIV-1-negative birth PCR test result, and infants infected in trapartum versus uninfected infants. Results: Of 49 infected infants with an HIV-1 birth PCR result, 12 (24.5%) [95% confidence interval (CI), 14-38] were presumed to have been infected i n utero and 37 (75.5%) were presumed to have been infected intrapartum. The estimated absolute overall transmission rate was 22.5%; this comprised 5.5 % (95% CI, 3-9) in utero transmission and 18% (95% CI, 13-24) intrapartum t ransmission. Intrapartum transmission accounted for 75.5% of infections. Hi gh maternal HIV-1 viral load (> median) was a strong risk factor for both i n utero [adjusted odds ratio (AOR) 5.8 (95% CI, 1.4-38.8] and intrapartum t ransmission (AOR 4.4; 95% CI, 1.9-11.2). Low birth-weight was associated wi th in utero transmission, whereas low maternal natural killer cell and CD4 T-lymphocyte percentages were associated with intrapartum transmission. Th e population-attributable fraction for intrapartum transmission associated with viral load > 10 000 copies/ml was 69%. Conclusions: Our results provide further evidence that most perinatal HIV-1 transmission occurs during labor and delivery, and that risk factors may d iffer according to time of transmission. Interventions to reduce maternal v iral load should be effective in reducing both in utero and intrapartum tra nsmission. (C) Lippincott Williams & Wilkins.