Objective: To investigate the possibility of reducing mother-to-child transmission (MTCT) of HIV-1 through breast-feeding by prophylactic antiretroviral (ARV) treatment of the infant during the breast-feeding period.

Design: An open-label, nonrandomized, prospective cohort study in Tanzania (Mitra).

Methods: HIV-1-infected pregnant women were treated according to regimen A of the Petra trial with zidovudine (ZDV) and lamivudine (3TC) from week 36 to 1 week postpartum. Infants were treated with ZDV and 3TC from birth to 1 week of age (Petra arm A) and then with 3TC alone during breast-feeding (maximum of 6 months). Counseling emphasized exclusive breast-feeding. HIV transmission was analyzed using the Kaplan-Meier survival technique. Cox regression was used for comparison with the breast-feeding population in arm A of the Petra trial, taking CD4 cell count and other possible confounders into consideration.

Results: There were 398 infants included in the transmission analysis in the Mitra study. The estimated cumulative proportion of HIV-1-infected infants was 3.8% (95% confidence interval [CI]: 2.0 to 5.6) at week 6 after delivery and 4.9% (95% CI: 2.7 to 7.1) at month 6. The median time of breast-feeding was 18 weeks. High viral load and a low CD4 T-cell count at enrollment were associated with transmission. The Kaplan-Meier estimated risk of HIV-1 infection at 6 months in infants who were HIV-negative at 6 weeks was 1.2% (95% CI: 0.0 to 2.4). The cumulative HIV-1 infection or death rate at 6 months was 8.5% (95% CI: 5.7 to 11.4). No serious adverse events related to the ARV treatment of infants occurred. The HIV-1 transmission rate during breast-feeding in the Mitra study up to 6 months after delivery was more than 50% lower than in the breast-feeding population of Petra arm A (relative hazard=2.61; P=0.001; adjusted values). The difference in transmission up to 6 months was significant also in the subpopulation of mothers with CD4 counts>or=200 cells/microL.

Conclusions: The rates of MTCT of HIV-1 in the Mitra study at 6 weeks and 6 months after delivery are among the lowest reported in a breast-feeding population in sub-Saharan Africa. Prophylactic 3TC treatment of infants to prevent MTCT of HIV during breast-feeding was well tolerated by the infants and could be a useful strategy to prevent breast milk transmission of HIV when mothers do not need ARV treatment for their own health.