The risk of transmitting human immunodeficiency virus type I (HIV-1) in breast milk is an established one, but its magnitude remains uncertain, and it is not clear whether this risk exceeds that of dying from formula-related diarrhea in developing countries. A randomized trial was started in Nairobi, Kenya, in 1992 to compare mortality and the risk of HIV infection in breast-fed and formula-fed infants. Initially 425 HIV-1–seropositive pregnant women who had received no antiretroviral study were enrolled in the trial. The infant was breast-fed in 212 instances and formula fed in 213. Randomization took place at about 32 weeks. End points were assessed after a median follow-up of 2 years in 401 mother-infant pairs. The study population was primarily of lower socioeconomic status. The median maternal age was 23 years.
The cumulative risk of HIV-1 infection was significantly higher for breast-fed infants. At 24 months it was 36.7 percent, compared with 20.5 percent for formula-fed infants. In the breast-feeding group, this practice accounted for 44 percent of all transmissions. A risk difference was apparent by age 6 weeks, and 87 percent of the overall difference in risk was manifest by age 12 months. Mortality curves did not differ significantly overall, but significantly more infants assigned to breast-feeding were infected or dead at 24 months (42 percent vs. 30 percent). The rate of HIV-1–free survival was 58 percent in the breast-fed group and 70 percent in the formula-fed group.
In Kenya, formula feeding substantially reduces the risk of transmitting HIV-1 to newborn infants, but formula remains too expensive for most infected women in sub-Saharan Africa. In addition to the direct cost of formula, antenatal viral testing is necessary and women require education in formula feeding.