The NCES data do not allow a distinction to be made among the three scenarios listed above. All three scenarios are consistent with a causal relation (at least in the sense of a trigger or proximate cause) between DPT and acute encephalopathy. The NCES data suggest that those children who experience acute neurologic illness might go on to develop chronic nervous system dysfunction or to die. Whether DPT can cause chronic nervous system dysfunction in those children who had experienced an acute neurologic illness within 7 days after receiving DPT is at issue in this report. The first scenario is consistent with a “causal” relation between DPT and chronic nervous system dysfunction; the third scenario is not. Whether the second scenario is consistent with a causal relation between DPT and chronic nervous system dysfunction is difficult to assess. If the children studied had experienced no acute neurologic illness other than that which occurred within 7 days after receiving DPT and the child experiences chronic nervous dysfunction, then the evidence is consistent with a causal relation between DPT and the chronic nervous system dysfunction. If the children had experienced other acute neurologic illness in addition to that following DPT, one could not say whether the evidence is consistent with a causal relation between DPT and the chronic nervous system dysfunction.

Because the NCES did not (and probably could not) rule out the possibility that only children with underlying brain or metabolic abnormalities react to stimuli such as DPT with acute neurologic illness, and no other studies establish or rule out such a possibility, the committee concludes that the evidence is insufficient to indicate whether or not DPT increases the overall risk in children of chronic nervous system dysfunction.

The NCES data are consistent with the possibility that some children without underlying brain or metabolic abnormalities might experience serious acute neurologic illness within 7 days after receiving DPT and that acute illness could have chronic nervous system sequelae. The NCES data also are consistent with the possibility that some children with underlying brain or metabolic abnormalities (which foster a “triggering” by DPT of an acute neurologic illness) might go on to develop chronic nervous system dysfunction due to a DPT-triggered acute illness. Therefore, the committee concludes that the balance of evidence is consistent with a causal relation between DPT and the forms of chronic nervous system dysfunction described in the NCES in those children who experience a serious acute neurologic illness within 7 days after receiving DPT vaccine. This serious acute neurologic response to DPT is a rare event. The estimated excess risk ranged from 0 to 10.5 per million immunizations (IOM, 1991). The committee stresses that this is not the strongest statement regarding causality; the evidence does not “establish” or “prove” a causal relation. The reader is referred to previous IOM reports for a more detailed explanation of these categories (IOM, 1991, 1994).



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement