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

In the absence of breast-feeding perinatal HIV transmission can occur either in utero or during the intrapartum period. Findings are presented from a prospective cohort study conducted to measure the proportion of HIV-1-infected infants infected in utero and intrapartum the relationship between transmission risk factors and time of transmission and the population-attributable fractions for maternal viral load. Findings are based upon data for 218 formula-fed infants of HIV-1-infected untreated mothers with known infection outcome and a birth HIV-1-positive DNA PCR test result. 49 infected infants with an HIV-1 birth PCR result 12 (24.5%) were presumed to have been infected in utero and 37 (75.5%) were presumed to have been infected intrapartum. In utero transmission was presumed to have occurred if the blood sample drawn within 72 hours of birth was HIV-1-positive by PCR. Intrapartum transmission was presumed to have occurred if the birth sample tested negative and a later sample was HIV-1-positive. The estimated absolute overall HIV transmission rate was 22.5%; 5.5% in utero transmission and 18% intrapartum transmission. High maternal HIV-1 viral load was a major risk factor for both in utero and intrapartum transmission. Low birth weight was associated with in utero transmission while low maternal natural killer cell and CD4+ T-lymphocyte percentages were associated with intrapartum transmission. The population-attributable fraction for intrapartum transmission associated with viral load higher than 10000 copies/ml was 69%.