Vaccination against H1N1 influenza with Pandemrix® during pregnancy and delivery outcome: a Swedish register study

Please cite this paper as: Kallen B, Olausson P. Vaccination against H1N1 influenza with Pandemrix (R) during pregnancy and delivery outcome: a Swedish register study. BJOG 2012;119:15831590. Objective To describe a large study on pregnancy outcome after vaccination against H1N1 during the 2009/10 pandemic. Design A cohort study of women vaccinated with Pandemrix (R) during pregnancy. Setting The Swedish Medical Birth Register was used for the analysis. Information on vaccination and pregnancy week when vaccination was made was obtained from antenatal care documents. Population All women who gave birth during 2009 and 2010 in Sweden. Methods Characteristics of the vaccinated women and their delivery outcome were compared with two groups of women: women without a known vaccination who gave birth in 2009/10 after 1 October 2009, and women who gave birth during 2009 before 1 October. Adjustment was made for year of delivery, maternal age, parity, smoking habits and body mass index. Outcome measures Stillbirth, congenital malformations, preterm birth, low birthweight, small for gestational age. Results A total of 18 612 vaccinated women having 18 844 infants were studied. The risk for stillbirth, preterm birth and low birthweight was lower than in the comparison groups whereas the risk for small for gestational age and a congenital malformation (after vaccination during the first trimester) did not differ from the comparison groups. No clear-cut explanation to the protective effect of vaccination was found. Conclusions Vaccination during pregnancy with Pandemrix (R) appeared to have no ill effects on the pregnancy. On the contrary, the rate of preterm birth and low birthweight was lower than expected, which agrees with some previous results.