In reply: We thank Kong and colleagues for their interest in our study.1 Our article highlighted the potential effect of influenza vaccination during pregnancy in reducing risk of pre‐term birth, as suggested by increased odds of vaccination among women birthing at 37 weeks or later compared with those birthing earlier than 28 weeks' gestation.1 We agree that one explanation of these results may be the potential protective effect that influenza vaccination during pregnancy has on pre‐term birth. This protective effect of influenza vaccines against a range of adverse birth outcomes has been noted in other studies.2 An alternative explanation, however, may be reverse causation, whereby women with longer periods of gestation (≥ 37 weeks) have an increased opportunity to be vaccinated. In this study, data on the timing of vaccination were not available, but we are in the process of making a submission to the data custodians of the Victorian Perinatal Data Collection to enable capture and examination of these data.
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