• Siri E Håberg, Lill Trogstad, Nina Gunnes, Allen J Wilcox, Håkon K Gjessing, Sven Ove Samuelsen, Anders Skrondal, Inger Cappelen, Anders Engeland, Preben Aavitsland, Steinar Madsen, Ingebjørg Buajordet, Kari Furu, Per Nafstad, Stein Emil Vollset, Berit Feiring, Hanne Nøkleby, Per Magnus, and Camilla Stoltenberg.
• Norwegian Institute of Public Health, Oslo, Norway. siri.haberg@fhi.no
• N. Engl. J. Med. 2013 Jan 24; 368 (4): 333-40.

BackgroundDuring the 2009 influenza A (H1N1) pandemic, pregnant women were at risk for severe influenza illness. This concern was complicated by questions about vaccine safety in pregnant women that were raised by anecdotal reports of fetal deaths after vaccination.MethodsWe explored the safety of influenza vaccination of pregnant women by linking Norwegian national registries and medical consultation data to determine influenza diagnosis, vaccination status, birth outcomes, and background information for pregnant women before, during, and after the pandemic. We used Cox regression models to estimate hazard ratios for fetal death, with the gestational day as the time metric and vaccination and pandemic exposure as time-dependent exposure variables.ResultsThere were 117,347 eligible pregnancies in Norway from 2009 through 2010. Fetal mortality was 4.9 deaths per 1000 births. During the pandemic, 54% of pregnant women in their second or third trimester were vaccinated. Vaccination during pregnancy substantially reduced the risk of an influenza diagnosis (adjusted hazard ratio, 0.30; 95% confidence interval [CI], 0.25 to 0.34). Among pregnant women with a clinical diagnosis of influenza, the risk of fetal death was increased (adjusted hazard ratio, 1.91; 95% CI, 1.07 to 3.41). The risk of fetal death was reduced with vaccination during pregnancy, although this reduction was not significant (adjusted hazard ratio, 0.88; 95% CI, 0.66 to 1.17).ConclusionsPandemic influenza virus infection in pregnancy was associated with an increased risk of fetal death. Vaccination during pregnancy reduced the risk of an influenza diagnosis. Vaccination itself was not associated with increased fetal mortality and may have reduced the risk of influenza-related fetal death during the pandemic. (Funded by the Norwegian Institute of Public Health.).

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