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Table 2 Incidence rate ratio (IRR) estimates, with associated 95 % confidence intervals (CIs), of febrile seizure in relation to the timing of influenza vaccination and infection

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

Exposure Period No. of person-days at risk No. of events Incidence rate (per 100 person-days) IRRa (95 % CI)
  Background periodd 976,010 739 0.08 1
  2 weeks before vaccination day 40,320 23 0.06 0.76 (0.50–1.43)
Vaccinationb Day of vaccination 2880 3 0.10 1.39 (0.46–4.35)
  1–3 days after vaccination 8640 13 0.15 2.00 (1.15–3.51)
  4–7 days after vaccination 11,520 7 0.06 0.81 (0.38 – 1.73)
  Background period 95,724 63 0.07 1
  2 weeks before influenza diagnosis 3962 9 0.23 3.96 (1.81–8.65)
Influenzac Day of influenza diagnosis 283 19 6.71 116.70 (62.81–216.90)
  1–3 days after influenza diagnosis 849 5 0.59 10.12 (3.82–26.82)
  4–7 days after influenza diagnosis 1132 1 0.09 1.50 (0.20–11.17)
  1. aAdjusted for calendar period (January–March, April–August, and September–December) and age in one-year bands
  2. bResults from self-controlled case series analysis, data from 656 Norwegian children born in 2006–2009 vaccinated with Pandemrix® and having 785 febrile seizure episodes during the total observation period (starting 180 days prior to vaccination or on day of birth [whichever came last], and ending 180 days after exposure or on the day of emigration or death [whichever came first])
  3. cResults from self-controlled case series analysis, data from 84 Norwegian children born in 2006–2009 diagnosed with pandemic influenza and having 97 seizures episodes during the total observation period (starting 180 days prior to vaccination or on day of birth [whichever came last], and ending 180 days after exposure or on the day of emigration or death [whichever came first])
  4. dAll other parts of the observation period