Vaccination against influenza during pregnancy protects pregnant women directly and indirectly protects their infants. Although trivalent inactivated influenza vaccines (IIV3) are safe and effective during pregnancy, quadrivalent inactivated influenza vaccines (IIV4) have not been studied in pregnant women or their infants. The study presents the findings of a randomized phase IV study in pregnant women to compare the immunogenicity and safety of IIV4 vs. IIV3. At weeks 20 to 32 of gestation, participants aged 18 years were randomly assigned in a 2:1 ratio to receive a single dose of IIV4 or IIV3. Hemagglutination inhibition (HAI) antibody titers increased in both groups by similar magnitudes between baseline and 21 days after vaccination for the two influenza A strains and a single B strain shared by IIV4 and IIV3. HAI titers were higher in IIV4 recipients than in IIV3 recipients for the additional B strain. HAI antibody titers for all strains were 1.5 1.9-fold higher in umbilical cord blood than in maternal blood at delivery in both groups, confirming active transplacental antibody transfer.

In both groups, the rates of solicited and unsolicited vaccine-related adverse events in mothers were comparable. All participants had live births, and there were no vaccine-related adverse events in newborns. These findings imply that IIV4 is as safe and immunogenic in pregnant women as IIV3, and that maternal immunization with IIV4 should protect newborns against influenza via passively acquired antibodies.