Objective. Acellular pertussis vaccines were introduced with the promise of an improved safety profile compared with whole-cell vaccines. In 1997–1998, Canada adopted 1 combination acellular pertussis vaccine, having previously used 1 particular combination whole-cell pertussis vaccine. We hypothesized that the change would result in a decrease in hospitalization rates for seizures and reports of hypotonic-hyporesponsive episodes (HHEs) temporally related to pertussis vaccination.

Methods. Active surveillance was performed between 1995 and 2001 by the Immunization Monitoring Program–Active monitors at 12 hospitals using standard case definitions. Seizures had to occur within 72 hours after immunization with a pertussis-containing vaccine or 5 to 30 days after immunization with measles-mumps-rubella vaccine. HHE episodes had to occur within 48 hours of receipt of a pertussis-containing vaccine. Poisson regression models were used to compare the average number of monthly admissions for seizures and HHEs before and after introduction of the acellular pertussis vaccine.

Results. We found a 79% decrease in febrile seizures associated with receipt of pertussis vaccine but no significant decrease in febrile seizures temporally related to measles-mumps-rubella between 1995–1996 and 1998–2001. There was a 60% to 67% reduction in HHEs associated with pertussis-containing vaccines between the same time periods, depending on case definition.

Conclusions. The risks of febrile seizures and HHEs after pertussis-containing vaccine declined significantly with the introduction of acellular pertussis vaccine in Canada. Active surveillance systems are important for detecting trends in uncommon adverse events after routine immunizations.

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