Background. Cytomegalovirus (CMV) is associated with morbidity and mortality in human immunodeficiency virus (HIV)-exposed infants. We assessed the effect of and relative contribution of breastfeeding to CMV acquisition among infants delivered by HIV-infected mothers. Methods. Between 1993 and 1998 pregnant, HIV-infected women in Nairobi, Kenya, were randomly assigned to breastfeed or formula-feed their infants in an HIV transmission study. Women were allocated equally between treatment arms, and the study was not blinded. The primary endpoint of this nested study was time to infant CMV infection. Results. CMV infection was assessed in 138 breastfed and 134 formula-fed infants. Baseline characteristics were similar between arms. Breastfed infants acquired CMV earlier than formula-fed infants (median age of acquisition, 4.26 vs 9.87 months; P <. 001) and had a higher 1-year probability of CMV infection (0.89 vs 0.69; P <. 001). Breastfeeding was associated with a 1.6-fold increased risk of infant CMV acquisition independent of infant HIV status (multivariable hazard ratio, 1.61; 95% confidence interval, 1.20-2.16; P =. 002). Approximately one third of CMV infections occurred during the peripartum period, with 40% acquired through breastfeeding and the remainder acquired through modes other than breast milk. Conclusions. Preventing CMV acquisition may be a priority for HIV-exposed infants, but there is a narrow window of opportunity for intervention. Approaches that reduce maternal cervical and breast milk CMV reactivation may help delay infant infection.
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