- ; Abstract: Interference from transplacental and breast milk antibodies may impede the performance of oral live vaccines. The effect of breastfeeding on the immunogenicity of Rotarix®, a two-dose oral monovalent rotavirus vaccine, was examined in a community-based trial in New Delhi, India. Four hundred mother–infant pairs were randomized into two equal groups. Infants were aged 6–7 weeks at enrollment. Mothers were encouraged to either breastfeed or to withhold breastfeeding during the 30 min prior to and after each vaccine dose was administered. We collected blood specimens from infants at enrollment and 4 weeks after the second vaccine dose. Blood and breast milk specimens were obtained from mothers at baseline and breast milk specimens were collected at the time of the second vaccine dose. Seroconversion was defined as infant serum anti-VP6 IgA antibody level of ≥20 IU/mL 4 weeks after the second vaccine dose and a ≥4-fold rise from baseline. There was no difference in the proportion who seroconverted between the two groups (26% vs 27%; p = 0.92). The levels of infant serum IgA, maternal serum and breast milk IgA and IgG anti-rotavirus antibodies predicted the anti-rotavirus IgA level in infants at end-study and explained approximately 10% of the variability of the immune response (r2 = 0.10, p