Effect of passive maternal antibody on influenza illness in children: a prospective study of influenza A in mother-infant pairs.

To determine the effect of passive antibody on the incidence of influenza in infants, infants born to mothers with serum antibody to influenza A (immune) and those born to mothers without evidence for this serum antibody (non-immune) were followed during the influenza H1N1 epidemic of 1979. Immune mothers had higher H1-specific antibody titers before the epidemic (P less than 0.001), were less frequently culture positive, showed fewer titer rises (P less than 0.001) and were less symptomatic than were nonimmune mothers. Infants of immune mothers had higher H1-specific passive antibody titers that correlated with their mother's antibody titers. Although infants of both groups showed no difference in incidence of influenza infection, infants of immune mothers had influenza symptoms that were delayed in onset (P = 0.02) and were of shorter mean duration compared with infants of nonimmune mothers. These findings suggest that passive maternal antibody delays the onset and decreases the severity of influenza disease in young infants.