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The high rates of morbidity and mortality in pregnant women at the beginning of the 2009 H1N1 influenza pandemic led to such women being given high priority for immunization. However, little is known about fetal safety of the influenza A (H1N1) vaccine. Investigators in Denmark recently took advantage of national birth and immunization registers to explore this issue.
The researchers identified a cohort of 53,432 live-born infants delivered between November 2, 2009, and September 30, 2010, of whom 6989 were exposed to monovalent inactivated AS03-adjuvanted influenza A (H1N1) vaccine. The median gestational age at the time of vaccine exposure was 174 days; 345 infants were exposed during the first trimester, and 6644 were exposed during the s…