Abstract
IMPORTANCE: Influenza and tetanus-diphtheria-acellular pertussis (Tdap) vaccinations during pregnancy offer protection to infants from infections. However, evidence about their effectiveness against hospitalization and emergency department (ED) visits associated with influenza and pertussis remains limited. OBJECTIVE: This study aimed to evaluate the association of maternal influenza and Tdap vaccinations with influenza- and pertussis-related hospitalizations and ED visits in infants younger than 6 months. DESIGN, SETTING, AND PARTICIPANTS: This population-based cohort study used the health care utilization databases from the Lombardy region of Italy. Pregnant individuals who received the influenza and Tdap vaccine among all live-birth pregnancies in 2018 to 2022 were included. Each vaccinated mother was matched with a nonvaccinated counterpart based on month and year of delivery, gestational age at birth, and pregnancy multiplicity. Analyses were performed from April 2024 to February 2025. EXPOSURES: Exposures of interest were influenza and Tdap vaccinations during pregnancy. MAIN OUTCOMES AND MEASURES: The primary outcomes were infant hospitalizations or ED visits due to influenza and pertussis. Cox regression models were fitted to estimate the hazard ratio (HR) of each outcome associated with the corresponding maternal vaccine. Vaccine effectiveness (VE) was calculated as VE = (1 - HR) × 100%. RESULTS: This study included 53 448 pregnant individuals who received the Tdap vaccine and 5347 who received influenza vaccine. The maternal vaccination coverage (ie, proportion of vaccinated pregnant individuals among those eligible) was 5359 (6.4%) for influenza and 70 119 (41.0%) for Tdap, respectively. Infants born to mothers who received the influenza and Tdap vaccine had a lower risk of hospitalization or ED visit for influenza (VE, 69.7%; 95% CI, 8.7%-90.0%) and pertussis (VE, 88.6%; 95% CI, 11.5%-98.5%), respectively. CONCLUSIONS AND RELEVANCE: This study found that maternal influenza and Tdap vaccinations were associated with reduced influenza- and pertussis-related hospitalization or ED visits in infants younger than 6 months. Given the low vaccination coverage, it is crucial to implement maternal vaccination campaigns to enhance infant health outcomes.