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Research Strategies for Assessing Adverse Events Associated withVaccines:: A Workshop Summary
diagnosed so differently among various studies that the terms becomealmost meaningless. People must agree to use the same diagnosticcriteria for the same conditions. Much education is necessary toget good background information. Some felt that this was possibleand that having standard definitions then assumes great importance.It was noted that even neurologists have not reached consensus onthe definitions of encephalopathy and encephalitis. However, in responseto the question of whether swine flu vaccine can cause GBS, a groupof neurologists was brought together to develop a definition thathas been generally accepted (Asbury and Cornblath, 1990; Asbury etal., 1978).
Adverse Events with Long Latencies
Previous discussions examined the potential utility of case reportsin assessing causality. This is particularly true for assessing thecausalities of adverse events that occur shortly after vaccination.Although anaphylaxis, for example, can occur without an obvious cause,the relation to vaccination in an individual case is easy to determineif it occurs within minutes of exposure to the antigen. In contrast,problems of assessing causality for adverse events with long latenciesfrom the time of exposure are many. The many discussions in the publishedliterature regarding the British National Childhood EncephalopathyStudy (NCES), including a recently released IOM report by the committeeto Study New Research on Vaccines (Institute of Medicine, 1994),show the lack of agreement in the scientific community on how bestto address these questions.
A participant noted that events with long latencies have traditionallybeen studied by the case-control method (as was done in the NCES)and that this will probably continue to be the primary way in whichthey are investigated. The selection of controls, however, is veryimportant. If the coverage rate for the vaccine is high, the requiredsample size could be large.
It was suggested that it might be useful to identify children whoparticipated in pre-marketing trials and follow them over the longterm.This is not currently done routinely, but it might be a worthwhileeffort, even though it would be difficult. Another participant commentedthat the unexposed group in a randomized trial will, if the vaccineis licensed and in general use in a few years, most likely becomevaccinated. In terms of looking at long latencies, there is not muchdifference between those who received a vaccine 18 years earlierand those who received it 21 years earlier.
Subacute sclerosing panencephalitis (SSPE) was mentioned as an exampleof the difficulty of dealing with long-latency events. A participantcommented on SSPE and multiple sclerosis; in the 1960s, both werebelieved to be associated with measles disease. Since the adventof the measles vaccine, there has been a 90 percent decrease in theincidence of SSPE, whereas there has been no change in the incidenceof multiple sclerosis. A participant commented that