system and coding practices are used?); for regulatory agencies (agenciesmay not use the same coding systems as the manufacturers); and fordata analysis and report generation (e.g., How many cases of adverseevent x have been reported with vaccine y?). The number of reports of a given adverse event identified withina system depends on the completeness of the search strategies andthe categories used to classify the adverse event. Many errors canbe made if search strategies are not standardized and validated.If the main purpose of passive surveillance systems is a signal detectionsystem (particularly for the detection of previously unrecognizedadverse events but also for the detection of increased numbers ofreports of a recognized adverse event), participants suggested thatsearch strategies should err in the direction of false positivesrather than false negatives.

Data Analysis

The use of summary statistics (e.g., how many reports of reactionx after vaccine y) from passive surveillance systems is a first step in assessingthe safety of vaccines, but limiting the analysis to those statisticscan lead to erroneous conclusions. The reported events occur in temporalrelation to vaccination, but not all events that occur within anappropriate interval from the time of vaccination are necessarilycaused by the vaccination. Although this might seem obvious, summarystatistics of reported events can be misinterpreted as evidence that the vaccine trulycauses the adverse event.

As a passive surveillance system, VAERS suffers problems of bothunderreporting and overreporting. Although health care professionalsare required to report some adverse events, specifically, those thatare covered by the no-fault component of the Vaccine Injury CompensationProgram, there are no provisions for enforcement. It is likely thatmany events that occur after the receipt of vaccines, like thosethat occur after the receipt of medications, go unreported. Peoplemay fail to report an adverse event because they are unaware of thereporting system, because they do not believe that the event is relatedto the vaccine, or because they know that the adverse event is anaccepted reaction (e.g., fever or sore arm) to that vaccine and considerit unnecessary to report what is already established. Because anyonecan report an adverse event to VAERS, it is possible that the patient,the manufacturer, and the health care provider could report the sameevent. If this duplication is not recognized and accounted for bythe system, the number of adverse events in the system would be spuriouslyhigh. Duplication is not the major source of overreporting. Seriousadverse events that occur after a vaccination may be reported evenif the reporting physician knows or strongly suspects another cause.

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