We studied a subgroup of 197 infants born to ART-naive HIV-1-infected mothers recruited in the ZVITAMBO study, which enrolled 14,000 mother-baby pairs between November 1997 and January 2000 in Harare, Zimbabwe [20] . ART prophylaxis for HIV-1-positive antenatal women was not available in the Harare public-sector during ZVITAMBO patient recruitment. The samples were consecutively drawn from two groups: 97 HIV-1positive mother/HIV-1-positive child pairs and 100 HIV-1positive mother/HIV-1-negative child pairs. Modes of infant HIV-1 transmission were determined using definitions adapted from Bryson and colleagues [21] and were described elsewhere [22] . Full methods for recruitment, baseline characteristics collection, laboratory procedures have been described elsewhere [20] . MTCT of HIV in the whole ZVITAMBO cohort occurred during the IU (22,9%), IP (48%) and PP periods (29.1%) [20] .