Obstet Gynecol

Maternal vaccination timing tied to infant flu outcomes

July 1, 2025

Study details: This large cohort study evaluated 245,498 infants born at Kaiser Permanente Northern California, comparing influenza incidence in infants whose mothers received influenza vaccination during pregnancy vs. those unvaccinated. Infants were followed from birth to 6 months, and vaccine effectiveness (VE) and trimester-specific effects were analyzed.

Results: Infants of vaccinated mothers had a lower incidence of influenza (0.12%) compared with those of unvaccinated mothers (0.30%). Adjusted VE for maternal vaccination during pregnancy was 44.4% (95% confidence interval [CI], 31.4–54.9%) against infant influenza. VE was 11.3% for first trimester, 51.5% for second trimester, and 59.3% for third trimester vaccination, with significantly greater protection when vaccination occurred in the second or third trimester (P=.02 and P<.001, respectively).

Clinical impact: These findings support current guidelines recommending influenza vaccination during any trimester of pregnancy, while highlighting enhanced infant protection when vaccination occurs in the second or third trimester. This strategy is especially vital, as infants under 6 months cannot be vaccinated and face a heightened risk of severe influenza-related complications.

Source:

Zerbo O, et al. (2025, June 26). Obstet Gynecol. Influenza Vaccination During Pregnancy and Infant Influenza in the First 6 Months of Life. https://pubmed.ncbi.nlm.nih.gov/40570349/