(A-340) Characterizing background risk of febrile seizure among pediatric patients during routine clinical care and vaccination

Background: Febrile seizure (FS) is an established adverse event associated with vaccination in young children. At the same time, young children have a background risk of FS due to infections that make them susceptible to high fever. Exposures during routine pediatric care may contribute to some of the FS risk observed during vaccination.

Objectives: To assess the background risk of FS in vaccinated and unvaccinated pediatric patients during routine well-child visits

Methods: We used IQVIA LRxDx Open Claims, a payor-agnostic, near real-time medical and pharmacy administrative claims database, to identify patients 0-4 years old with FS within one day of a well-child visit (WCV) and/or vaccination (COVID-19, influenza, or all others) from 2021-2024. We used Current Procedural Term codes to identify WCV, and National Drug Codes for vaccines. We conducted separate self-controlled case series (SCCS) to assess risk of FS associated with WCV alone and vaccine exposures, both overall and stratified by age, varying risk windows (0-1 day, 0-7 days).

Results: Overall, the rate of FS after a WCV alone was 9.00 (Confidence Interval [CI]: 8.23-9.77) per 100,000 patients, similar to any immunization alone (9.05 [CI: 7.48-10.63]); the rates of FS after COVID-19 vaccine (7.17 [CI: 4.80-9.55]) and influenza vaccine (4.84 [CI: 3.43-6.26]) were lower. Rates were higher after COVID-19 and influenza vaccines among pediatric patients 2-3 compared to 0-1 years old, but rates were higher for WCV and any immunization alone among patients 0-1 years old. In the SCCS, WCV without vaccination had an incidence rate ratio (IRR) of 1.25 (CI: 1.10-1.41) while WCV with vaccination had an IRR of 1.74 (CI: 1.59-1.89) among those 0-1 years old. The first dose of any vaccination (IRR=1.63 [CI: 1.52-1.76]) and COVID-19 vaccination (IRR=1.53 [CI: 1.12-2.10]) had statistically significant elevated risk, whereas influenza vaccination did not (IRR=1.17 [CI: 0.98-1.39]).

Conclusions: Routine pediatric care through WCV is associated with FS risk even without vaccination. Vaccine studies on FS risk in pediatric patients may need to additionally account for the effect of exposures related to routine care.