Influenza Vaccination Given at Least 2 Weeks Before Delivery to Pregnant Women Facilitates Transmission of Seroprotective Influenza-specific Antibodies to the Newborn

Background: Pregnant women and infants are at higher risk of complications secondary to influenza infection. Immunization during pregnancy facilitates protection of the neonates through passive transfer of maternal antibodies. Methods: This was a cross-sectional study performed during the post-H1N1 pandemic winter season of 2010/2011 in Geneva, Switzerland. We measured antibody titers against the seasonal influenza A H1N1, H3N2 and B 2010/2011 strains by hemagglutination inhibition in the umbilical cord blood of newborns born to vaccinated and nonvaccinated mothers. Seroprotection was defined as a hemagglutination inhibition titer 40. Results: A total of 188 women were enrolled, 101 of whom had been vaccinated with a nonadjuvanted influenza vaccine (all during the second or third trimester) and the other 87 had not. Among newborns of vaccinated women, 84-86% showed seroprotective levels depending on the strain. In comparison, seroprotection rates were significantly lower in babies of nonvaccinated women (29-33%, P < 0.001). Adjusting for various confounding factors and applying multivariate regression analysis, vaccination during pregnancy 2 weeks before delivery increased geometric mean titers in umbilical cord blood 5-17 times and seroprotection rates 5.8-34.4 times, depending on the strain and the interval between vaccination and delivery. Vaccinating pregnant women only 2-4 weeks before delivery was still more effective than no vaccination at all (geometric mean titers increased 6.8-11.1 times and seroprotection rates increased 5.8-34.4 times compared with nonvaccinated women). Conclusions: Influenza vaccination at any time during the second and third trimester of pregnancy, but at least 15 days before delivery, confers seroprotection to many neonates.