Background: Vaccines are important to public health, but because of the way they are manufactured, their mechanism of action, and their indicated population, careful monitoring of their adverse events is necessary. Canada has a national surveillance system that collects reports on adverse events that may be associated with vaccine administration. Sensitivity is one of the tools used with surveillance systems to study the extent and characteristics of reporting of a surveillance system. To date, the sensitivity of the Canadian system has not been assessed.
Purpose: To assess the sensitivity of the Canadian Adverse Event Following Immunization Surveillance System (CAEFISS).
Methods: Based on specific adverse events following immunization (AEFI) and vaccines chosen for the study, a thorough literature search was completed to find the best source which identifies expected rates of AEFI. Studies used were assessed based on quality and sample size. The expected rates of AEFI, in combination with public health estimates of vaccine coverage rates, were used to estimate the expected number of reports. The reports provided the actual number of events used to calculate the sensitivity. Sensitivity was compared based on year of administration, age group, and type of AEFI.
Results: The overall sensitivity of the CAEFISS varied from 1.0% to 136.6% for various AEFI for the years 1997 to 2008. For influenza the sensitivity was found to be 93.6% and 136.3% for GBS and anaphylaxis respectively. For DTaP, the rates were found to be 15.0%, 1.0%, and 21.2% for anaphylaxis, HHE, and seizures respectively, and for MMR the rates were 16.5%, 52.7%, and 12.7% in relation to anaphylaxis, thrombocytopenia, and seizures respectively.
Conclusions: This is the first assessment of the sensitivity of the CAEFISS, and this study found that the system has reasonable ability to detect AEFI on a national level. CAEFISS had comparable senstivity to other vaccine reporting systems. Many of the AEFI had sensitivity values higher than the 5%-10% range traditionally seen in other passive surveillance systems related to adverse events. The greatest variation of sensitivity was seen between vaccines. Rarity and timing of the AEFI may also impact the sensitivity. Variation of sensitivity and the variation found in the sensitivity analysis lend to the further development and implementations of case definitions for rarer adverse events, especially anaphylaxis. Further research of other factors that impact reporting is necessary
