Introduction: Febrile seizures are a heterogeneous entity. Within this category are febrile seizures that occur in close temporal proximity to DTP or MMR. However, it is not known whether the long-term outcomes of children with febrile seizures following vaccination are similar to those of children with febrile seizures unrelated to immunization.

Methods: To address this question we focused on three outcome measures: 1) the risk for subsequent seizures; 2) the proportion of children with subsequent febrile or afebrile seizures; and 3) the proportion of children who went on to develop epilepsy. We utilized chart abstracted data from a large, previous Vaccine Safety Datalink study of seizures following immunization. The exposed population was defined as children with DTP exposure within 7 days prior to febrile seizure, or MMR exposure 8-21 days prior to febrile seizure. The unexposed population was defined as children with febrile seizures temporally unrelated to vaccination. Outcome data concerning subsequent seizure events, or diagnosis of epilepsy, were taken from abstracted medical records. Cox proportional hazards was used to assess the risk for subsequent seizures among the exposed compared to the unexposed children.

Results: There were a total of 562 febrile seizures among children at the four HMO sites. Of these, 42 followed immunization with either DTP or MMR, and 521 were temporally unrelated to vaccination. Among all children, 82 (15%) had at least one subsequent febrile seizure, and 13 (2%) had at at least one subsequent afebrile seizure. Among children followed at least six months, 15% of exposed children, and 13% of the unexposed children had at least one additional seizure. Among children followed for one year, none of the exposed children were diagnosed with epilepsy, while 3% of the unexposed children were diagnosed with epilepsy. Using Cox proportional hazards analysis, exposed children were less likely than unexposed children to have subsequent seizures (RR 0.65;95%CI 0.62,0.94).

Discussion: Our results suggest that children with febrile seizures following vaccination are not at increased risk for subsequent seizures or for epilepsy compared to children with febrile seizures unrelated to immunization.