Background: In many countries, influenza vaccination is routinely recommended during any stage of pregnancy, yet uptake remains low, particularly in the first trimester. This is thought to be due to maternal concerns regarding vaccine safety.

Objective: To evaluate the safety of influenza vaccination in the first trimester of pregnancy.

Methods: In a 4-year retrospective cohort study using probabilistic record linkage of administrative health data, we established a population-based cohort of 2391 women vaccinated in first trimester and 68 447 never vaccinated women with a date of conception between 2012 and 2015 in Western Australia. We estimated the relative risk (RR) of perinatal health outcomes among first trimester vaccinated women as compared to never vaccinated women using log-binomial logistic regression following a propensity score matched (PSM) analyses (2391 vaccinated women matched with 9564 never vaccinated women).

Results: First trimester vaccination was not associated with increased risk of stillbirth (RR 1.18, 95% confidence interval [CI] 0.64, 2.19), small for gestational age (RR 0.96, 95% CI 0.83, 1.11) or preeclampsia (RR 0.97, 95% CI 0.74, 1.28). The risk of spontaneous birth at 32-36 weeks was higher in first trimester vaccinated women compared with never vaccinated women (RR 1.40, 95% CI 1.11, 1.77). Vaccination was associated with a 10-19% increase in the risk of gestational diabetes (RR 1.18, 95% CI 0.94, 1.49), premature rupture of membranes (RR 1.10, 95% CI 0.82, 1.48), and threatened preterm labour (RR 1.19, 95% CI 0.90, 1.59).

Conclusions: With exception to spontaneous preterm birth, findings suggest that first trimester vaccination is not associated with adverse maternal and foetal outcomes. Results can be used to support patient and provider-level vaccine decision making during first trimester.