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 <14 or ≥14 weeks gestation.

Results: Sixty-five women with SAB had received H1N1 vaccination at 6-19 weeks gestation. The IRR of SAB for the risk period 1-28 days was 1.03 (95% confidence interval [CI] 0.55-1.93). Among the 147,294 live births and 1354 stillbirths, maternal H1N1 vaccine receipt at <14 weeks gestation was associated with significantly reduced likelihood of small for gestational age (SGA) birth (OR 0.72, 95% CI 0.61-0.84) and birth defect (OR 0.46, 95% CI 0.22-1.00), whereas receipt at ≥14 weeks gestation was associated with significantly reduced likelihood of stillbirth (OR 0.63, 95% CI 0.46-0.86), prematurity (OR 0.90, 95% CI 0.83-0.97), low birth weight (OR 0.81, 95% CI 0.74-0.88), and SGA birth (OR 0.90, 95% CI 0.84-0.97).

Conclusions: H1N1 vaccination during pregnancy did not increase risk of SAB or adverse fetal outcomes.

Keywords: H1N1 virus; MF59 adjuvant; Pregnancy; Taiwan; Vaccine safety.