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

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. 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. 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). 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.