the question now is not whether the measles vaccine can prevent SSPE;obviously it can, by preventing disease from wild-type measles virusinfection. Still in question is whether there are rare cases of SSPEcaused by the vaccine. Although technical advances are making itpossible to isolate virus from the brains of patients with SSPE andtype the RNA as being of the vaccine strain or the wild-type, thefailure to isolate vaccine-strain virus does not prove that the vaccinecannot cause SSPE. As was articulated by the IOM committees (Instituteof Medicine, 1991, 1994a) it is impossible to prove a negative. However,typing an isolated virus from a patient with SSPE as vaccine strainwould be important data to suggest that measles vaccine can cause SSPE. This is hypothetical at present.
Adverse events that are multifactorial in etiology, for example,insulin-dependent diabetes mellitus, provide particular challenges.In these instances, the vaccine might serve as an additional stimulusto a genetic background that predisposes a child toward the condition.It was mentioned that as more and more genes for diseases are identified,more conditions that fall into this category may be identified.
Once causality between a condition and a vaccine is established,epidemiologic methods could be used to study why some people whoreceived, for example, influenza vaccine got GBS and others did not.What is different about those who develop the disease or condition?But even if one could identify risk factors for adverse events, itmay not be feasible or practical to screen for them prior to immunization.
A participant commented that there are a number of suspicions aboutpredisposing conditions that might be exacerbated by vaccines (forexample, arthritis and hepatitis B vaccine). Such examples shouldbe defined and looked at in LLDBs. It was suggested that generalresearch projects designed primarily to study the pathophysiologyor genetics of conditions such as diabetes might include vaccineissues as part of their study designs.
A participant stressed that it is important to understand the mechanismsof the adverse events. It would be a mistake to address the problemof safety only from the moment of licensure. Basic laboratory researchon possible immunopathologic or infectious sequelae of vaccines,in addition to surveillance and epidemiologic studies, is needed.