Febrile seizures after 2009 influenza A (H1N1) vaccination and infection: a nationwide registry-based study.

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.