Safety of inactivated monovalent pandemic (H1N1) 2009 vaccination during pregnancy: A population-based study in Taiwan

Abstract Background Pregnant women were prioritized for H1N1 vaccination during the 2009–2010 pandemic. Safety concerns exist with vaccinating pregnant women, particularly in their first trimesters. Methods We linked computerized data on H1N1 vaccination, National Health Insurance, and Taiwan Birth Registry and identified events of spontaneous abortions (SABs) and all singleton births that occurred/delivered during November 1, 2009–September 30, 2010. The observation period for each case of SAB (6–19 weeks gestation) was divided into period at risk (1–28 days after vaccination) and control periods (the remaining person-days until SAB). The self-controlled case series method for truncated observational periods assessed the incidence rate ratio (IRR) of SAB during the 1–28 days compared with those in the control period. The case-control design matched each case of adverse fetal outcomes to up to 10 controls on fetal sex and year/month of pregnancy onset, and calculated matched odds ratio (OR) on H1N1 vaccination at