COVID-19 vaccine initiation in pregnancy and risk for adverse neonatal outcomes among United States military service members, January-December 2021

Background: The safety of primary series COVID-19 vaccine exposure in pregnancy has been well-studied; however, no research to date has been conducted among United States (US) military service members, a unique population with specific vaccination requirements for active duty service and early COVID-19 vaccine access. Methods: This retrospective cohort study leveraged data from the Department of Defense Birth and Infant Health Research program to identify live births among active duty US military service members in calendar year 2021. Administrative military personnel data, immunization files, and medical encounter records were used to develop study variables and determine COVID-19 vaccine receipt in pregnancy. Cox and modified Poisson regression models estimated hazard (HR) and risk ratios (RR), respectively, with 95 % confidence intervals (CI) for vaccine receipt and selected neonatal outcomes; models were adjusted for baseline characteristics using inverse probability of treatment weighting and further adjusted for SARS-CoV-2 infection in pregnancy. Results: There were 7184 singleton live births included for analysis, of which 2867 (39.9 %) were among service members exposed to their first COVID-19 vaccine dose in pregnancy and 4317 (60.1 %) among service members unexposed to any COVID-19 vaccine during or prior to pregnancy. Baseline differences between exposed and unexposed service members (e.g., age, race and ethnicity, marital status, occupation) were fully attenuated after applying weights. COVID-19 vaccine initiation in pregnancy was not associated with preterm birth (637 weeks' gestation; adjusted HR: 1.02, 95 % CI: 0.83-1.26), small for gestational age (610th percentile; adjusted HR: 1.01, 95 % CI: 0.78-1.30), low birthweight (62500 g; adjusted HR: 1.01, 95 % CI: 0.80-1.28), or neonatal intensive care unit admission (adjusted RR: 0.90, 95 % CI: 0.75-1.07). Conclusion: Primary series COVID-19 vaccine exposure in pregnancy was common in this military cohort. Vaccine receipt was not associated with increased risk for any adverse outcome under study, substantiating findings from existing literature.