(355) Tetanus, Diphtheria, Pertussis (Tdap) Vaccination During Pregnancy and Initiation of Human Milk Feeding

Background: Since 2012, the United States (US) Advisory Council on Immunization Practices has recommended that Tdap vaccine be given during every pregnancy, ideally between 27 and 36 gestational weeks, to reduce pertussis morbidity and mortality in infants. Although studies have demonstrated the safety of Tdap during pregnancy, there are no published studies that assess the risk of adverse lactation outcomes following antepartum Tdap vaccination.

Objectives: To assess the association between antepartum Tdap vaccination and initiating human milk feeding.

Methods: Using electronic health record (EHR) data from two large health care systems in the US, we identified prenatal care patients who had a live birth between 2014-2023, could be linked to their infant who had ≥1 infant well-visit before 60 days of life, and did not have Tdap vaccination before 27 gestational weeks. Tdap vaccination was assessed using EHR-based vaccine registries, medical and pharmacy claims, or state immunization information system data. The exposure was Tdap vaccination between 27 gestational weeks and delivery and the comparator was no Tdap vaccination during pregnancy. The outcome was no human milk feeding initiation versus any human milk feeding and was assessed using data stored in structured fields in EHR from routine infant up to 12 months of age and postpartum visits. For multiple gestations, the outcome was classified as any human milk feeding if at least one infant met the outcome criteria. We used Poisson regression with robust variance to estimate adjusted risk ratios (aRR) and 95% confidence intervals (CI) adjusting for site and calendar year to account for temporal trends. We further adjusted for maternal age, race and ethnicity, public insurance, neighborhood-level poverty index, education and median household income, self-reported smoking, pre-pregnancy body mass index, number of pre-pregnancy health care visits and medication dispensings, prenatal care adequacy index, parity, and multiple gestation. We used multiple imputation by chained equations to address missing covariate values.

Results: Among the 36,244 pregnancies meeting inclusion criteria, 84.2% received Tdap. Among those who received Tdap during pregnancy, 4.0% did not initiate human milk feeding. Among those who did not receive Tdap during pregnancy, 5.7% did not initiate human milk feeding. Following Tdap vaccination, the year and site-adjusted aRR of not initiating human milk feeding was 0.7 (95%CI: 0.6, 0.8). After adjusting for all covariates, the aRR was 0.8 (95% CI: 0.7, 0.9).

Conclusions: Tdap vaccination during pregnancy was not associated with an increased risk of breastfeeding non-initiation. The study demonstrates the utility of using routinely collected data on lactation in EHR to study vaccine safety.