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 maternally 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 reverse transcriptase-polymerase chain reaction 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 antibody 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 old. 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 with 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.