Safety assessment post-licensure.

The objectives of post-licensure surveillance are to identify rare or novel adverse events, estimate their rate of occurrence and distinguish those that are causally related to vaccination. Evidence of causality is based on biological plausibility supported by laboratory evidence of vaccine involvement and/or a statistically significant excess of events in the post-vaccination period. Passive reporting systems have methodological limitations, particularly for ascertaining reliable adverse event rates and investigating causal relationships. In Canada, an active surveillance system (IMPACT) has been established in sentinel paediatric hospitals where nurses scrutinise all admissions for conditions conforming to a defined set of adverse events. While ascertainment of events resulting in hospital admission should be complete, causality is difficult to assess without information on the incidence of such events outside the post-vaccination risk period. The use of computerised data bases which link clinical events with immunisation records have allowed the risk of events such as convulsions attributable to DTP, MMR and Hib vaccines to be defined. The record linkage method promises to be a useful method for routine surveillance of vaccine safety.