Influenza Vaccination During Pregnancy and Infant Influenza in the First 6 Months of Life

OBJECTIVE To evaluate the effectiveness of influenza vaccination during pregnancy against influenza in infants during their first 6 months of life. METHODS We conducted a cohort study among pregnant individuals enrolled in Kaiser Permanente Northern California (KPNC) and their infants. We followed all infants from birth until the first occurrence of a polymerase chain reaction test result positive for influenza, the infant reached age 6 months, death, disenrollment from KPNC, or the end of the study on December 31, 2022. We used Cox regression to compare the hazard of influenza in infants whose mothers were vaccinated against influenza during pregnancy with those whose mothers were unvaccinated. Hazard ratios (HRs) were adjusted for calendar time, maternal sociodemographic and comorbidities. Vaccine effectiveness was calculated as 100% (1-adjusted HR). We evaluated the association between vaccination and infant influenza outcomes any time during pregnancy and by trimester of vaccination. RESULTS Of the 245,498 infants included in the study, 46.0% were born to vaccinated mothers. The incidence of influenza was lower among infants of vaccinated mothers than unvaccinated mothers (0.12% vs 0.30%). After adjusting for covariates, vaccination during pregnancy was associated with a reduction in infant influenza in any clinical setting by 44.4% (95% CI, 31.4-54.9%). Vaccination during the first trimester was associated with a reduction in infant influenza by 11.3%, a reduction of 51.5% during the second trimester, and a reduction of 59.3% during the third trimester. The differences in vaccine effectiveness estimates that compared the first and second trimesters (P=.02) and compared the first and third trimesters (P<.001 were statistically significant. CONCLUSION Influenza vaccination during pregnancy was associated with a reduction in infant influenza infection by 44.4%. The reduction in infant influenza infection was greater when vaccination occurred in the second or third trimester, compared with the first trimester.