Introduction: DTP and MMR appear to be associated with a transiently increased risk for febrile, but not afebrile, seizures. It is not known if this risk is modified or increased among children with underlying neurologic problems (such as cerebral palsy), or children at risk for problems(such as low birth weight or prematurity). To address this issue, we utilized the Vaccine Safety Datalink, a population-based study of adverse events following childhood immunization in four health maintenance organizations.

Methods: Automated information systems were used to identify children with one or more of the following conditions: structural CNS abnormalities; congenital anomalies; cerebral palsy; degenerative diseases; or perinatal conditions such as birth asphyxia or prematurity. Tracking systems were used to collect information on routine immunizations, and automated databases identified children with diagnostic codes indicative of possible seizures. At one HMO, seizure cases were ascertained through ancillary data sources, while at two HMOs a sample of cases were reviewed due to the large number of potential seizures. Validated seizures were classified by type(including simple and complex febrile seizures, afebrile, or other seizure types), and by incident (first ever) or recurrent seizure type. Cox proportional hazards analysis, adjusted for age, time, and HMO, assessed the association between immunization and incident seizures among impaired/at-risk children, and among normal children. An interaction term was used to assess whether the association between vaccination and seizures was modified among the impaired/at risk group.

Results: Among 13,303 neurologically impaired/at-risk children there were 151 validated incident seizures. Of these, 36 were febrile, and 49 were afebrile seizures. Impaired/at-risk children were 5.6 times more likely to have a seizure (RR 5.6;95%CI 4.5,7.0) than normal children. DTP was associated with a 1.7 fold elevated risk for seizures in the 30 days following vaccination (RR 1.7;95%CI 1.2,2.3). However, this risk from DTP was not modified nor elevated among impaired or at-risk children. Of the 151 incident seizures, 15 occurred within 30 days of a DTP vaccination, and only 1 occurred within 30 days of an MMR vaccination.

Discussion: Children with neurologic impairments have an elevated risk for seizures compared to neurologically normal children. Regardless of neurologic status, there is a modestly increased risk for seizures in the month following DTP vaccination.