The risk of major structural birth defects associated with seasonal influenza vaccination during pregnancy: a population-based cohort study

Objective: Seasonal inactivated influenza vaccine (IIV) is recommended during pregnancy to protect both mothers and infants from severe infection. Most studies have evaluated the risk of major structural birth defects in infants associated with prenatal administration of pandemic IIV. Our aim was to estimate the risk in infants associated with prenatal administration of seasonal IIV. Design: Retrospective population-based observational study Setting: Western Australia Population or sample: All pregnant women with a singleton birth from 2012-2016 Methods: 125,866 singleton births were linked to the state’s registers for congenital anomalies and a state prenatal vaccination database. We estimated prevalence ratios (PR) of any major structural birth defect and defects by organ system. Inverse probability treatment weighting factored for baseline probability for vaccination. Bonferroni correction was applied to account for multiple comparisons. Main outcome measures: Prevalence ratios by vaccination and major structural birth defects categories or specific birth defects diagnosed within one month of birth or within six years of life. Results: 3.9% of births had a major structural birth defect. IIV exposure during the first trimester was not associated with diagnosis of any major structural birth defect diagnosed within one month of birth (PR 0.98, 95% CI: 0.77, 1.28) or within six years of life (PR 1.02, 95% CI: 0.78, 1.35). We identified no increased risk in specific birth defects associated with IIV. Conclusions: Results suggest there is no association between maternal influenza vaccination and risk of major structural birth defects. These results support the safety of IIV administration during pregnancy.