The role of breast milk antirotavirus immunoglobulin A (IgA) and trypsin inhibitors in limiting the acquisition of rotavirus infection during the initial 5 days of life was evaluated among 42 exclusively breast‐fed hospital‐born infants, 22 of whom experienced rotavirus infection. The mean concentrations of antirotavirus IgA (ELISA Units) in the breast milk of mothers of the 22 rotavirus‐infected neonates was 130.4 ± 46.4; the corresponding value in 20 noninfected neonates was 384.3 ± 328.3 (P < 0.001). Similarly, the trypsin inhibitory capacity (μmols/mt/ml) of breast milk in the rotavirus‐infected group was significantly lower (0.109 ± 0.095) than that in the noninfected group (0.376 ± 0.191; P < 0.001). The trypsin inhibitory capacity of milk showed an inverse correlation with infant stool tryptic activity (P < 0.01).

Our results indicate that the acquisition of rotavirus infection during the early neonatal period depends on the concentrations of antirotavirus IgA and trypsin inhibitors in human milk and that protection is mediated by high levels of these antiviral factors.