The impact of timing of maternal influenza immunisation on infant antibody levels at birth

Pregnant women and infants are at an increased risk of severe disease after influenza infection. Maternal immunisation is a potent tool to protect both of these at‐risk groups. Whilst the primary aim of maternal influenza vaccination is to protect the mother, a secondary benefit is the transfer of protective antibodies to the infant. A recent study using the tetanus, diphtheria and acellular pertussis (Tdap) vaccine indicated that children born to mothers immunised in the second trimester of pregnancy had the highest antibody titres compared to children immunised in the third trimester. The aim of the current study was to investigate how timing of maternal influenza immunisation impacts infant antibody levels at birth. Antibody titres were assessed in maternal and cord blood samples by both IgG‐binding ELISA and haemagglutination inhibition assay (HAI). Antibody titres to the H1N1 component were significantly higher in infants born to mothers vaccinated in either the second or third trimesters than infants born to unvaccinated mothers. HAI levels in the infant were significantly lower when maternal immunisation was performed less than four weeks before birth. These studies confirm that immunisation during pregnancy increases the antibody titre in infants. Importantly antibody levels in cord blood were significantly higher when mother was vaccinated in either trimester two or three, though titres were significantly lower if the mother was immunised less than 4 weeks before birth. Based on this data, seasonal influenza vaccination should continue to be given in pregnancy as soon as it becomes available.