Update: Vaccine Side Effects, Adverse Reactions,
Despite these methodologic difficulties, the National Childhood Encephalopathy Study (NCES) and other controlled epidemiologic studies have provided evidence that DTP can cause acute encephalopathy (64-68). This adverse event occurs rarely, with an estimated risk of zero to 10.5 episodes per million DTP vaccinations (68). A detailed follow-up of the NCES indicated that children who had had a serious acute neurologic illness after DTP administration were significantly more likely than children in the control group to have chronic nervous system dysfunction 10 years later. These children with chronic nervous system dysfunction were more likely than children in the control group to have received DTP within 7 days of onset of the original serious acute neurologic illness (i.e., 12 {3.3%} of 367 children vs. six {0.8%} of 723 children) (69).
After reviewing the follow-up data, IOM concluded that the NCES provided evidence of an association between DTP and chronic nervous system dysfunction in children who had had a serious acute neurologic illness after vaccination with DTP. The committee proposed three possible explanations for this association. First, the acute neurologic illness and subsequent chronic nervous system dysfunction might have been caused by DTP. Second, DTP might trigger an acute neurologic illness and subsequent chronic nervous system dysfunction in children who have underlying brain or metabolic abnormalities. Such children might experience similar chronic dysfunction in the absence of DTP vaccination if other stimuli (e.g., fever or infection) are present. Third, DTP might cause an acute neurologic illness in children who have underlying brain or metabolic abnormalities that would inevitably have led to chronic nervous system dysfunction even if the acute neurologic illness had not developed (6). IOM concluded that the NCES data do not support one explanation over another.
According to IOM, the balance of evidence was consistent with a causal relationship between DTP and some forms of chronic nervous system disorders in children who had developed an acute neurologic disorder after receiving DTP. However, IOM also concluded that the results were insufficient to determine whether DTP increases the overall risk for chronic nervous system dysfunction in children.
A subcommittee of the National Vaccine Advisory Committee (NVAC) also reviewed the study and concluded that the results were insufficient to determine whether DTP administration before the acute neurologic event influenced the potential for neurologic dysfunction 10 years later (Ad hoc Subcommittee of the NVAC, unpublished data, 1994). ACIP concurs with this evaluation.
Although the NCES examined and reported risk for the 7 days after DTP vaccination, the increased risk for serious acute neurologic illness occurred primarily during the first 3 days after DTP administration (64). Thus, if an association between DTP and chronic encephalopathy exists, the risk is primarily in the first 3 days after DTP vaccination.
Among a subset of children who were participating in the NCES and who had infantile spasms, both DTP and DT vaccination appeared either to precipitate early manifestations of the condition or to lead to its identification by parents (70). IOM reviewed this and other studies and concluded that neither vaccine causes the illness (71,72).
Sudden infant death syndrome (SIDS) is listed on death certificates as the cause of death for 5,000-6,000 infants (ages 0-364 days) each year in the United States. Because the peak incidence of SIDS for infants occurs at 2-4 months of age, many instances of a close temporal relation between SIDS and receipt of DTP are reported.