Protective effect of maternal immunization on birth outcomes: A data linkage study

Objective To explore the impact of maternal immunization on adverse pregnancy outcomes including preterm birth (PTB) and stillbirth. Methods The authors performed a data linkage study for women who delivered a singleton baby between January 2017 and May 2021. They used Poisson models to estimate incidence rates of adverse pregnancy outcomes and Cox proportional hazards models to estimate hazard ratios (HRs) with 95% confidence intervals (CIs), accounting for the time‐dependent nature of the exposure and adjusting for confounders. Results This study included 10 938 women who received at least one vaccine, and 4029 unvaccinated women. Influenza vaccine was associated with a significant reduction in stillbirth (adjusted HR [aHR], 0.55 [95% CI, 0.33–0.94]), but not in PTB (aHR, 0.92 [95% CI, 0.77–1.10]). Pertussis vaccine was associated with a significant reduction in PTB (aHR, 0.78 [95% CI, 0.64–0.94]) and a similar point estimate for reduction in stillbirth (aHR, 0.59 [95% CI, 0.31–1.10]), although not significant. Conclusion Reductions in PTB and stillbirth associated with maternal immunization suggest possible protective effects beyond pathogen‐specific protection. These findings may strengthen justification for scaling up maternal immunization in low‐income settings where there remains a high burden of these adverse pregnancy outcomes. Synopsis Vaccines, as an existing licensed and safe product, could be considered as a scalable intervention through which pathogen‐agnostic mechanisms may provide opportunities to improve pregnancy outcomes.