In addition to over- and underreporting, it is difficult to assessthe meaning of absolute numbers of reports for several reasons. Considerthe following examples:
Many people who experience an adverse event have more than one symptomand each symptom is given a separate code. A child who experiencesa fever, a sore arm, and vomiting is listed as having three distinctadverse events. Thus, the number of adverse events recorded by thesystem is far greater than the number of people who have experiencedan adverse event.
One would expect more reports of adverse events following receiptof diphtheria and tetanus toxoids and pertussis vaccine (DPT) vaccinethan following receipt of measles, mumps, and rubella vaccine simplybecause each child receives more doses of DPT than measles, mumps,and rubella vaccine.
One would expect to find more reports of sudden infant death syndrome(SIDS) associated with a lot of vaccine in use in the winter monthsthan with lots in use during summer because the background rate (andtherefore the expected number of coincidental reports) of SIDS ishigher in winter than in summer.
Childhood vaccines are frequently given in combination, and eachadverse event occurring following receipt of each vaccine given onthe same day is given a separate code. Summary statistics from apassive surveillance system such as VAERS will show a great manyreports of a sore arm following receipt of the oral polio vaccine(OPV). What that summary statistic does not indicate is that thechild concurrently received other vaccines that were responsiblefor the sore arm. In fact, there is no causality assessment madefor the reports in VAERS, because the system was not designed tosupport such an assessment.
These examples describe the expected complications of passive surveillancesystems. Reporting and analysis of summary statistics require carefulconsideration of and explanations for these factors.
Passive surveillance systems can be used to establish patterns ofadverse event reporting and to monitor changes in those patternsas vaccines change (different lots, different formulations, differentantigens), or as the populations that receive particular vaccineschange. Although case reports can provide important information regardingcausality under specific circumstances, the participants generallyagreed that passive surveillance systems best serve to detect signalsor warnings that there might be a problem rather than to answer questionsabout causation. These signals can lead to hypotheses about causality,which can then be tested by other methods, such as epidemiologicor laboratory studies.