Previous studes have indicated that in 1–2 year old children receiving a first dose, receipt of the measles, mumps, rubella and varicella (MMRV) combination vaccine is associated with a twofold increased risk of febrile seizures 7–10 days after immunization in when compared with separately administered same-day MMR and varicella vaccine. This study will examine genomic indicators to pinpoint suseptibility to this increased risk.