Infection with influenza virus during pregnancy poses a significant risk of complications for both mother and fetus. During the H1N1 2009 pandemic, pregnant women constituted one of the priority groups for vaccination in many countries, creating a need for close monitoring of the safety of the vaccine in pregnant women. We present findings from an analysis of a cohort of pregnant women (N=267) from a prospective, observational, post-authorization safety study of the AS03-adjuvanted split virion H1N1 (2009) pandemic vaccine. There were 265 known pregnancy outcomes with 261 live births, four spontaneous abortions with no congenital anomalies, and no stillbirths. There were six live births with congenital anomalies, of which one was diagnosed before vaccination. A total of 247 women (94.6%), of whom four had twin pregnancies, delivered at term, and 14 women (5.4%), of whom two had twin pregnancies, delivered preterm (between Weeks 24 and 36 of gestation), with three of them (1.1%) occurring before 32 weeks (very preterm). Twenty-one neonates (8.1%) had a low birth weight (<2.5 kg), of whom nine (3.5%) were term neonates. The prevalence of all outcomes was in line with the expected rates. The adverse events reported were consistent with the events anticipated to be reported by this study population. No adverse events of special interest were reported. The results of this analysis suggest that exposure to the AS03 adjuvanted H1N1 (2009) vaccine during pregnancy does not increase the risk of adverse pregnancy outcomes including spontaneous abortion, congenital anomalies, preterm delivery, low birth weight neonates, or maternal complications. Although limited in size, the fully prospective nature of the safety follow-up of these women vaccinated during pregnancy is unique and offers an important degree of reassurance for the use of the AS03 adjuvanted H1N1 (2009) vaccine in this high risk group for H1N1 infection.
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