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Abstract
BackgroundDuring 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.MethodsThe 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.ResultsThe 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).ConclusionsIn 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.
Highlights
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
In Norway, pandemic influenza vaccination was offered from October 19th, 2009
Most children received the vaccine in the late half of the main pandemic wave (Fig. 1), and 98.1 % of all pandemic vaccine doses were given before January 1st, 2010
Summary
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. Our aim was to estimate the risk of febrile seizure in children after exposure to pandemic influenza vaccination or infection. Febrile seizure is estimated to occur in 2 to 5 % of all children 6 through 60 months of age, and is defined as seizures accompanied by fever (≥38 °C) without central nervous system infection [1] Several viruses, such as human herpesvirus 6 and influenza viruses, have been associated with increased risk of febrile seizures [2,3,4,5,6]. During the 2009 influenza A (H1N1) pandemic, a monovalent pandemic strain vaccine containing the oil-in-water adjuvant AS03 (Pandemrix®) was offered free of charge to the Norwegian population, with the national vaccination coverage among children reaching 54 % [15]. Our results are likely to apply to more severe influenza infections
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