title: Febrile seizures after 2009 influenza A (H1N1) vaccination and infection: a nationwide registry-based study.
abstract: BACKGROUND
  During the 2009 influenza A (H1N1) pandemic, a monovalent pandemic strain vaccine containing the oil-in-water adjuvant AS03 (Pandemrix®) was offered to the Norwegian population. The coverage among children reached 54%. Our aim was to estimate the risk of febrile seizure in children after exposure to pandemic influenza vaccination or infection.
  
  
  METHODS
  The study population comprised 226,889 children born 2006-2009 resident in Norway per October 1st, 2009. Febrile seizure episodes were defined by emergency hospital admissions / emergency outpatient hospital care with International Classification of Diseases, Version 10, codes R56.0 or R56.8. The self-controlled case series method was applied to estimate incidence rate ratios (IRRs) in pre-defined risk periods compared to the background period. The total observation window was ± 180 days from exposure day. Among 113,068 vaccinated children, 656 (0.6%) had at least one febrile seizure episode.
  
  
  RESULTS
  The IRR of febrile seizures 1-3 days after vaccination was 2.00 (95% confidence interval [CI]: 1.15-3.51). In the period 4-7 days after vaccination, no increased risk was observed. Among the 8172 children diagnosed with pandemic influenza, 84 (1.0%) had at least one febrile seizure episode. The IRR of febrile seizures on the same day as a diagnosis of influenza was 116.70 (95% CI: 62.81-216.90). In the period 1-3 days after a diagnosis of influenza, a tenfold increased risk was observed (IRR 10.12, 95% CI: 3.82 - 26.82).
  
  
  CONCLUSIONS
  In this large population-based study with precise timing of exposures and outcomes, we found a twofold increased risk of febrile seizures 1-3 days after pandemic influenza vaccination. However, we found that pandemic influenza infection was associated with a much stronger increase in risk of febrile seizures.
authors.affiliation: Norwegian Institute of Public Health, Nydalen, PO Box 4404, Oslo, N-0403, Norway. inger.johanne.bakken@fhi.no.
authors.forename: Inger Johanne
authors.initials: IJ
authors.lastname: Bakken
authors.affiliation: Norwegian Institute of Public Health, Nydalen, PO Box 4404, Oslo, N-0403, Norway. kari.modalsli.aaberg@fhi.no.|National Centre for Epilepsy, Oslo University Hospital, Oslo, Norway. kari.modalsli.aaberg@fhi.no.
authors.forename: Kari Modalsli
authors.initials: KM
authors.lastname: Aaberg
authors.affiliation: Norwegian Institute of Public Health, Nydalen, PO Box 4404, Oslo, N-0403, Norway. sara.ghaderi@fhi.no.
authors.forename: Sara
authors.initials: S
authors.lastname: Ghaderi
authors.affiliation: Norwegian Institute of Public Health, Nydalen, PO Box 4404, Oslo, N-0403, Norway. nina.gunnes@fhi.no.
authors.forename: Nina
authors.initials: N
authors.lastname: Gunnes
authors.affiliation: Norwegian Institute of Public Health, Nydalen, PO Box 4404, Oslo, N-0403, Norway. lill.iren.trogstad@fhi.no.
authors.forename: Lill
authors.initials: L
authors.lastname: Trogstad
authors.affiliation: Norwegian Institute of Public Health, Nydalen, PO Box 4404, Oslo, N-0403, Norway. per.magnus@fhi.no.
authors.forename: Per
authors.initials: P
authors.lastname: Magnus
authors.affiliation: Norwegian Institute of Public Health, Nydalen, PO Box 4404, Oslo, N-0403, Norway. sirieldevik.haberg@fhi.no.
authors.forename: Siri Eldevik
authors.initials: SE
authors.lastname: Håberg
mesh_terms: D002675:Child, Preschool; D004632:Emergency Medical Services / Q000706:statistics & numerical data; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology / Q000472:pathogenicity; D007252:Influenza Vaccines / Q000009:adverse effects*; D007251:Influenza, Human / Q000453:epidemiology / Q000517:prevention & control* / Q000821:virology; D008297:Male; D009664:Norway / Q000453:epidemiology; D012042:Registries; D003294:Seizures, Febrile / Q000453:epidemiology* / Q000209:etiology; D014611:Vaccination / Q000009:adverse effects*
chemical_list: D007252:Influenza Vaccines; C556153:pandemrix