Maternal antibodies against influenza in cord blood and protection against laboratory-confirmed influenza in infants.

BACKGROUND Studies that correlate maternal antibodies with protection from influenza A or B virus infection in young infants in areas with prolonged influenza circulation are lacking. METHODS We conducted a prospective, observational study to evaluate the effects of maternal-transferred antibodies against influenza A and B viruses against laboratory-confirmed influenza in a cohort born over 24 months. Cord blood samples were retrieved at birth and infants were actively followed for the first 6 months of life. Nasal swabs were collected and tested for influenza A and B by RT-PCR whenever an illness episode was identified. Cord blood samples were tested by the hemagglutination inhibition (HAI) assay to viruses that circulated during the follow-up period. RESULTS 1162 infants were born to 1140 recruited women: 1092 (94%) infants completed 6 months of follow-up. Proportions of cord blood with HAI antibodies titers ≥40 against A(H1N1), A(H3N2), B/Victoria and B/Yamagata were 31%, 24%, 31% and 54%, respectively. Only 4% of women had maternal influenza vaccination. Cord blood antigen-specific HAI titers ≥40 were found to correlate with protection from infection only for influenza B/Yamagata. No influenza B virus infection occurred in infants ≤60 days of life. Proportional hazards analysis showed that a cord blood HAI titer of 40 was associated with 83% (95% confidence interval: 44%, 95%) reduction in the risk of influenza B/Yamagata infections compared to a cord blood titer <10. CONCLUSIONS We documented that maternal immunity against influenza B/Yamagata was conferred to infants within the first 6 months of life.