INFLUENZA vaccination during pregnancy significantly reduces the risk of influenza in infants during their first six months of life, with the strongest protection observed when vaccination occurs in the second or third trimester, according to a large cohort study.
Researchers evaluated data from over 245,000 mother-infant pairs enrolled in Kaiser Permanente Northern California. Infants were followed from birth through six months of age or until they tested positive for influenza via PCR, disenrolled, died, or reached the study’s end in December 2022. The investigators used adjusted Cox regression models to assess the association between maternal influenza vaccination during pregnancy and infant influenza incidence.
Among the infants studied, 46% were born to vaccinated mothers. The rate of influenza was lower in infants of vaccinated mothers compared with those of unvaccinated mothers, 0.12% versus 0.30%, respectively. After adjustment for key maternal variables and calendar time, maternal vaccination was associated with a 44.4% reduction in infant influenza (95% CI, 31.4–54.9%).
Timing of vaccination mattered. When administered during the second trimester, vaccine effectiveness was 51.5%, and during the third trimester, it rose to 59.3%. In contrast, vaccination in the first trimester offered only an 11.3% reduction in infant influenza risk. The differences in effectiveness between the first and later trimesters were statistically significant (P=0.02 and P<0.001, respectively). These findings reinforce the value of maternal influenza immunization, not only for protecting pregnant individuals, but also for extending protection to newborns during their first six months of life, when they are too young for vaccination themselves. Timing of vaccination may further enhance this passive immunity. Reference: Zerbo O, Modaressi S, Goddard K, et al. Influenza Vaccination During Pregnancy and Infant Influenza in the First 6 Months of Life. Obstetrics & Gynecology. 2025. doi:10.1097/AOG.0000000000005986.
