Breast-feeding during primary maternal human immunodeficiency virus infection and risk of transmission from mother to infant.

Examination of breast-fed infants of the complete cohort of Australian women whose primary human immunodeficiency virus (HIV) infection occurred postpartum allows for an estimate of risk of transmission of HIV. Ten women with no other risk factors were infected via blood transfusion postpartum. They breast-fed for up to 9 months; 2 of their infants were infected. Another woman, who shared needles for intravenous drug use, seroconverted 6-10 months post-partum. She breast-fed for 14 months. Retrovirus was visualized in the cellular and cell-free fraction of her milk by electron microscopy. Infection in her infant was confirmed at 12 months. Thus, 3 of the 11 babies at risk became infected, providing an estimate of risk of 27% for breast-feeding during primary maternal infection (95% confidence interval, 6-61%). These data establish the association of primary maternal HIV infection and breast-feeding with a high risk of transmission to infants.