• Jonas F Ludvigsson, Henric Winell, Sven Sandin, Sven Cnattingius, Olof Stephansson, and Björn Pasternak.
• Karolinska Institutet, Stockholm, and Örebro University Hospital, Örebro, Sweden, School of Medicine, University of Nottingham, Nottingham, United Kingdom, and Celiac Disease Center, Columbia University College of Physicians and Surgeons, New York, New York (J.F.L.).
• Ann. Intern. Med. 2020 Oct 20; 173 (8): 597-604.

BackgroundThere are concerns that influenza vaccine exposure during pregnancy may be associated with increased risk for autism spectrum disorder (ASD).ObjectiveTo examine the risk for ASD in offspring of mothers who were vaccinated against influenza A(H1N1)pdm09 ("swine flu") during pregnancy.DesignPopulation-based cohort study using nationwide registers.SettingSeven health care regions in Sweden.ParticipantsLive births between October 2009 and September 2010, with follow-up through December 2016. In total, 39 726 infants were prenatally exposed to H1N1 vaccine (13 845 during the first trimester) and 29 293 infants were unexposed.MeasurementsCox regression was used to estimate hazard ratios (HRs) for the primary outcome, ASD, before and after adjustment for potential confounders. The secondary outcome was autistic disorder (AD).ResultsMean follow-up was 6.7 years in both unexposed and exposed children. During follow-up, 394 (1.0%) vaccine-exposed and 330 (1.1%) unexposed children had a diagnosis of ASD. In adjusted analyses, prenatal exposure to H1N1 vaccination was not associated with a later diagnosis of ASD (adjusted HR [aHR], 0.95 [95% CI, 0.81 to 1.12]) or AD (aHR, 0.96 [CI, 0.80 to 1.16]). The 6-year standardized cumulative incidence difference between the unexposed and exposed children was 0.04% (CI, -0.09% to 0.17%) for ASD and 0.02% (CI, -0.09% to 0.14%) for AD. Restricting the analysis to vaccination in the first trimester of pregnancy did not influence risk estimates (aHR, 0.92 [CI, 0.74 to 1.16] for ASD and 0.91 [CI, 0.70 to 1.18] for AD).LimitationData on H1N1 influenza infection are lacking.ConclusionThis large cohort study found no association between maternal H1N1 vaccination during pregnancy and risk for ASD in the offspring.Primary Funding SourceSwedish Research Council.

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