The incidence of childhood and adolescent seizures in the UK from 1999 to 2011: A retrospective cohort study using the Clinical Practice Research Datalink

Background: In postmarketing vaccine surveillance, adverse events observed in a vaccinated population are compared to the number expected based on a background incidence rate. The background rate should be accurate and obtained from a population comparable to the one vaccinated. Such rates are often not available.

Methods: The incidence rate of generalised convulsive, febrile and afebrile seizures was estimated in individuals born after 01-January-1998 and aged between 2 months and 15 years of age using the UK Clinical Practice Research Datalink (1999-2011).

Results: The study population consisted of 1532,992 individuals (4917,369 person years (PY) of follow up). A total of 28,917 generalised convulsive seizure events were identified during follow-up, the overall incidence rate was 5.88 per 1000PY. Age specific rates increased sharply from 4/1000PY at 2 months of age, peaked at 19/1000PY at 16 months and decreased until approximately 6 years of age at which point they became relatively stable at 2/1000PY. 67% of GCSs were categorised as febrile: 56% using Read codes, 11% using free text. Febrile seizures accounted for the age trend in GCS, with rates peaking at 16.1/1000PY at 16 months of age while afebrile seizure rates remained relatively stable across all ages (24 seizures per 1000PY). Analysis by first occurrence of febrile seizure showed a similar pattern, comparable to published studies on the incidence of seizures in childhood.

Discussion: The rates reported in this study could be used in the postmarketing surveillance of infant vaccines. However, given the variation across strata, and the potential underascertainment of seizure events presenting to A&E, care must be taken when interpreting and using these rates.