receiving DPT vaccine. This serious acute neurologic response to DPT is a rare event. The excess risk has been estimated to range from 0 to 10.5 per million immunizations (IOM, 1991). The evidence does not “establish” or “prove” a causal relation. The evidence remains insufficient to indicate the presence or absence of a causal relation between DPT and chronic nervous system dysfunction under any other circumstances. That is, because the NCES is the only systematic study of long-term dysfunctions after DPT, the committee can only comment on the causal relation between DPT and those long-term dysfunctions under the conditions studied by the NCES. In particular, it should be noted that the long-term dysfunctions associated with DPT followed a serious acute neurologic illness that occurred in children within 7 days after receiving DPT.
In 1991, the Institute of Medicine (IOM) issued a report entitled Adverse Effects of Pertussis and Rubella Vaccines. Congress mandated that report and the study that led to it as part of the 1986 National Childhood Vaccine Injury Compensation Act (Public Law 99-660). The report reviewed the scientific literature bearing on the causal relation between diphtheria and tetanus toxoids and pertussis vaccine (DPT) and 18 putative serious adverse effects of DPT. Perhaps the most controversial of all the relations studied was that between DPT and encephalopathy. The committee concluded, “The evidence is consistent with a causal relation between DPT vaccine and acute encephalopathy, defined in the controlled studies reviewed as encephalopathy, encephalitis, or encephalomyelitis” (IOM, 1991, p. 118). The committee concluded that the range of excess risk of acute encephalopathy following DPT immunization was on the order of 0.0 to 10.5 per million immunizations.
On the basis of the available data at the time of that report, the committee further concluded, “There is insufficient evidence to indicate a causal relation between DPT vaccine and permanent neurologic damage ” (IOM, 1991, p. 118). The committee examined the relation between DPT vaccine and various types of seizures as well, because seizures are considered by some to be a component of encephalopathy and because data existed for these separate analyses. The committee concluded (a) that the evidence indicated a causal relation between DPT and febrile seizures, (b) that the evidence did not indicate a causal relation between DPT and afebrile seizures, and (c) that there was insufficient evidence to indicate a causal relation between DPT and epilepsy. The inability to determine causality between DPT and permanent neurologic damage centered on the incompleteness of the preliminary findings reported from the 10-year follow-up study of the National Childhood Encephalopathy Study (NCES) (Madge et al., 1990). Those data have since been reported in full (Madge et al., 1993;