Background:  Breastfeeding pattern and duration are associated with postnatal HIV acquisition, but their relative contribution has not been reliably quantified.

Methods:  Pooled data from 2 studies with a similar approach, 1 in urban West Africa, where breastfeeding cessation at 4 months was recommended and where exclusive breastfeeding was rare (Ditrame Plus, DP); and 1 in rural South Africa, where promotion of safer breastfeeding practices resulted in high rates of exclusive breastfeeding, but with longer breastfeeding duration (Vertical Transmission Study, VTS). The 18-month HIV postnatal transmission was assessed by Kaplan-Meier analysis. Censoring with (to assess effect of mode of breastfeeding) and without (to assess effect of breastfeeding duration), breastfeeding was cessation considered as a competing event.

Results:  Overall, of 1195 breastfed infants, not HIV-infected perinatally, 83% VTS and 38% DP children were still breastfed at age 6 months. By age 3 months, 66% of VTS children were exclusively breastfed since birth and 55% of DP infants predominantly breastfed (breastmilk + water-based drinks). Postnatal-transmission risk at 18-month (95%CI) in VTS was double that in DP:  9% (7 to 11) and 5% (3 to 8), respectively (p = 0.03). However, once the duration of breastfeeding was allowed for in a competing risk analysis assuming that all children would have been breastfed for 18 months, the postnatal-transmission risk would be 14% (10 to18) in VTS and 16% (8 to 28) in DP (p = 0.32). The 18-month postnatal-transmission risk was 3.9% (2.3 to 6.5) among children breastfed for less than 6 months, and 8.7% (6.8 to 11.0) among those breastfed for more than 6 months; crude hazard ratio (HR): 2.1 (1.2 to 3.7), p = 0.02; adjusted HR 1.8 (0.9 to 3.4), p = 0.06 (adjustment on confounders including maternal CD4 count). In individual analyses on postnatal-transmission rates for specific breastfeeding durations, risks among children exclusively breastfed were very similar to those in children predominantly breastfed for the same period. Children exposed at least once to solids during the first 2 months of life were 2.9 (1.1 to 8.0) times more likely to be infected postnatally than children never exposed to solids this early (adjusted competing risk analysis, p = 0.04).

Conclusions:  Breastfeeding duration is a major determinant of postnatal HIV transmission, and safe alternatives to breastmilk beyond 6 months are urgently needed. The postnatal-transmission risk was not different in exclusively and predominantly breastfed children, and we confirm the negative effects of mixed breastfeeding with solids.