Immunization Monitoring Program, Active: a model of active surveillance of vaccine safety.

In Canada since 1993 the nation's passive system for reporting postimmunization adverse events has been supplemented by the active surveillance of inpatients at 10 to 12 pediatric referral centers, a system referred to as the Immunization Monitoring Program, Active. Participating centers are located from coast-to-coast and receive referrals from every province and territory. Approximately 20 percent of the population aged 0 to 12 years lives in the immediate vicinity of these centers. Nurse monitors at each center search for numerous target conditions, including postimmunization adverse events and vaccine-preventable infections. Vaccine safety observations have included (1) a substantial decrease in the risk of the development of febrile seizures and hypotonic-hyporesponsive episodes since the country switched from whole-cell to acellular pertussis-containing vaccines, (2) no evidence for encephalopathy resulting from the latter vaccines, (3) a generally benign outcome with postimmunization thrombocytopenia cases, and (4) an unexpectedly high rate of disseminated bacille Calmette-Guérin infections among aboriginal infants. Concomitant disease surveillance has been important for sustaining the surveillance system because few postimmunization adverse events require hospital admission.