Objectives: Influenza is associated with an increased risk for serious illness, hospitalization, and death in pregnant women and young infants. Our aim was to estimate the effectiveness of a quadrivalent inactivated influenza vaccine (QIV) in pregnant women and their infants during 2019-2020.
Methods: A QIV vaccine was offered to pregnant women followed in a maternity hospital. Women were contacted weekly during the influenza season and asked about symptoms. Polymerase chain reaction testing in pharyngeal samples was offered to pregnant women and infants with influenza-like illness. A Bayesian beta-binomial model was used.
Results: We studied 636 pregnant women (406 vaccinated and 230 unvaccinated) and 474 infants (281 of mothers vaccinated in pregnancy and 193 of unvaccinated mothers). Using a Bayesian beta-binomial model, it was estimated that influenza vaccination of pregnant women reduced their logit to develop laboratory-confirmed influenza by -4.2 (95% CI -3,7 - 4,7) and the logit of their infants to develop laboratory-confirmed influenza by -4.2 (95% CI -3.6, -4.9). The QIV effectiveness against laboratory-confirmed influenza was 43.5% in pregnant women and 31.4% in infants.
Conclusion: Maternal influenza vaccination with QIV in pregnancy reduced the odds of pregnant women and their infants to develop influenza.
Clinical trial registration: www.clinicaltrials.gov identifier is NCT04723771.
Keywords: Effectiveness; infants; influenza vaccination; pregnant woman; quadrivalent influenza vaccine.