Impact of maternal influenza vaccination during pregnancy on the incidence of acute respiratory illness visits among infants

Objective: To determine whether influenza vaccination of pregnant women prevents visits for respiratory illness in their infants born during the influenza season. Design: Retrospective matched cohort study. Setting: Four managed care organizations in the United States. Patients: A total of 41 129 infants (3160 and 37969 born to vaccinated and unvaccinated mothers, respectively) born between 1995 and 2001. Main Exposure: Maternal influenza vaccination. Infants were considered exposed if their gestational age at birth was at least 30 weeks, if the time from maternal vaccination to birth was at least 28 days, and if they were exposed to at least 14 days of the influenza season. Main Outcome Measures: Incidence of acute respiratory illnesses (outpatient, emergency department, and inpatient settings combined) and incident rate ratios (IRRs) for infants exposed and unexposed to maternal vaccination during the following 4 periods: peak influenza, respiratory syncytial virus predominant, periseasonal, and summer weeks. The time to the first acute respiratory illness during peak influenza weeks was also assessed. Results: During the peak influenza weeks, infant visit rates were 15.4 and 17.1 per 100 person-months for exposed and unexposed infants, respectively (IRR, 0.90; 95% confidence interval, 0.80-1.02). Adjusted IRRs for the 4 periods found a protective effect of infant female sex, whereas Medicaid status and maternal high-risk status increased infant visit rates. Maternal influenza vaccination did not reduce visit rates during any of the 4 time periods (IRR for peak influenza season, 0.96; 95% confidence interval, 0.86-1.07) and did not delay the onset of first respiratory illness. Conclusion: We were unable to demonstrate that maternal influenza vaccination reduces respiratory illness visit rates among their infants.