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Rotavirus is the main cause of severe gastroenteritis and diarrhea in infants and young children less than 5 years of age. Potential impact of breast-feeding on the efficacy and immunogenicity of human rotavirus G1P vaccine was examined in this exploratory analysis.Healthy infants (N = 3994) aged 6–14 weeks who received 2 doses of human rotavirus vaccine/placebo according to a 0–1 or 0–2 month schedule were followed for rotavirus gastroenteritis during 2 epidemic seasons. Rotavirus IgA seroconversion rate (anti-IgA antibody concentration ≥20 mIU/mL) and geometric mean concentrations were measured prevaccination and 1–2 months post-dose 2. Vaccine efficacy against any and severe rotavirus gastroenteritis was analyzed according to the infants being breast-fed or exclusively formula-fed at the time of vaccination.Antirotavirus IgA seroconversion rate was 85.5% (95% confidence interval [CI]: 82.4–88.3) in breast-fed and 89.2% (95% CI: 84.2–93) in exclusively formula-fed infants; geometric mean concentrations in the respective groups were 185.8 U/mL (95% CI: 161.4–213.9) and 231.5 U/mL (95% CI: 185.9–288.2). Vaccine efficacy was equally high in breast-fed and exclusively formula-fed children in the first season but fell in breast-fed infants in the second rotavirus season. During the combined 2-year efficacy follow-up period, vaccine efficacy against any rotavirus gastroenteritis was 76.2% (95% CI: 68.7–82.1) and 89.8% (95% CI: 77.6–95.9) and against severe rotavirus gastroenteritis 88.4% (95% CI: 81.6–93) and 98.1% (95% CI: 88.2–100) in the breast-fed and exclusively formula-fed infants, respectively.The difference in immunogenicity of human rotavirus vaccine in breast-fed and exclusively formula-fed infants was small. Vaccine efficacy was equally high in breast-fed and exclusively formula-fed children in the first season. Breast-feeding seemed to reduce slightly the efficacy in the second season.