689 - Risk of Febrile Seizures following Measles Vaccination Among Children 6-59 months in a Federally Qualified Health Care System, 2008-2024

Publication Number: 689.6337

Joshua T. Williams, Denver Health, Denver, CO, United States; Kate Kurlandsky, Denver Health, Denver, CO, United States; Hannah Cruz, Denver Health, Denver, CO, United States; Amy B. Stein, Denver Health, Denver, CO, United States; simon J.. Hambidge, Denver Health Community Health Services, Denver, CO, United States

Background: Up to 5% of children experience febrile seizures (FS); prior work has associated measles vaccines with a brief but significantly increased risk of FS following vaccination. Ongoing surveillance for measles vaccine associated (MVA) FS in diverse settings is critical to maintaining public confidence in vaccines and relevant to parent-clinician vaccine discussions.

Objective: To estimate the relative risk of FS following measles vaccination among children 6-59 months old presenting to care at a Federally Qualified Health System (FQHS) in Denver, CO from July 2008 to April 2024.

Design/Methods: This was a retrospective cohort study of electronic health record data. The outcome was FS, defined using ICD-9/10 codes obtained from inpatient or ER visits, which have been shown to have high positive predictive value in these settings. The exposure was a measles-containing vaccination (e.g., measles, or measles-mumps-rubella, or measles-mumps-rubella-varicella vaccine) 0-14 days prior to FS diagnosis. MVA cases were chart reviewed to confirm temporality and extract relevant covariates. Children with epilepsy, meningitis, or metabolic disturbances were excluded. We calculated descriptive analyses, crude rates of MVA and non-MVA FS, and adjusted relative risks (aRR) of FS.

Results: 52,828 children were included (Table 1), of whom 407 (0.8%) had a FS; of all FS, 18 were chart-confirmed MVA FS (4.4%). Over the study period, the crude incidence rate of MVA FS was 2.00 (95% CI 1.18, 3.15) per 100,000 person-days, with the crude incidence rate of MVA-FS peaking in 2009-10 and 2020-21 (Figure 1). The crude incidence rate of non-MVA FS was 0.70 (95% CI 0.63, 0.77) per 100,000 person-days, which remained stable over time (Figure 1). In adjusted analyses, the aRR for MVA-FS was 2.86 (95% CI 1.72, 4.44) in the 0-14 days after administration, referent to the risk in children who were not vaccinated for measles in that time. Sex and age group were also significantly associated with an adjusted relative risk of FS (Table 2).

Conclusion(s): Between 2008 and 2024, few children 6-59 months old presented to the ER or inpatient ward of a FQHS with a FS; of these children, fewer than 1 in 20 had a MVA FS. These findings contribute to ongoing surveillance efforts regarding measles vaccine safety and help contextualize the risks of FS following measles vaccination for parents and clinicians. Future work should explore the higher incidence of MVA FS observed during the COVID-19 pandemic.

Table 1. Demographic characteristics of the study cohort (children 6-59 months old empaneled in a Federally Qualified Healthcare System between July 2008 and April 2024).
PAS_MMR_FS_20241104_Table 1.pdf

Table 2. Unadjusted and adjusted* relative risks of febrile seizures in children 6-59 months old in a federally qualified healthcare system (July 2008- April 2024).
PAS_MMR_FS_20241104_Table 2.pdf

Figure 1. Yearly crude incidence rate of measles vaccine associated and non measles vaccine associated febrile seizures per 100,000 person-days within a Federally Qualified Healthcare System (2009-2024).
PAS_MMR_FS_20241104_Figure 1.pdf