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Adverse Events Associated with Childhood Vaccines: Evidence Bearing on Causality
cine Injury Table, encephalopathy has been strictly defined (U.S. Department of Health and Human Services, 1992). The Vaccine Injury Table defines the vaccines and adverse events that are covered under the National Vaccine Injury Compensation Program. Health care providers must report the occurrence of an adverse event listed in the table if it falls within the specified latencies from vaccination. The box in Chapter 10 includes the current Vaccine Injury Table and Aids to Interpretation. Some proposed changes are reproduced in this chapter in the box entitled Changes Proposed by DHHS in the Definition of Encephalopathy in the Aids to Interpretation of the Vaccine Injury Table.
This definition of acute and chronic encephalopathy is useful because of its precision, and the committee considered this proposed definition as it reviewed the evidence. As is made clear throughout the report, the evidence reviewed by the committee varied greatly in both the quality and quantity of clinical details provided. Very few studies provided enough detail to ascertain whether the cases of encephalopathy reported meet the criteria proposed in the Vaccine Injury Table. The committee read and considered all reports of encephalopathy, regardless of the extent of documentation of the adverse event. Although a 24-hour period for the duration of stupor or coma associated with encephalopathy is a widely accepted and reasonable standard based on a unit of time (a day), the committee felt that it is not an absolute. However, this distinction regarding the time period did not affect the final conclusions regarding encephalopathy.
The occurrence of encephalopathy in a child does not imply a particular severity or duration of illness, nor does it indicate that a child will have irreversible brain injury. Many children do recover from serious neurologic illnesses and therefore may not have permanent neurologic sequelae (Institute of Medicine, 1991). The annual incidence of encephalitis in Olmsted County. Minnesota, from 1950 to 1981 was 7.4 per 100,000 people (Beghi et al., 1984). The incidence in children less than age 1 year was 22.5, in children between age 1 and 4 years it was 15.2, and in children between ages 5 and 9 years it was 30.2 per 100,000. There was a case fatality rate of 3.8 percent.
Aseptic meningitis refers to inflammation of the meninges, not of the brain. It can result from a variety of infectious, toxic, chemical, or physical agents. No bacterial organism can be identified in or isolated from the cerebrospinal fluid, but serologic studies often implicate a viral etiology. Mumps virus and polioviruses can cause aseptic meningitis. The annual incidence of aseptic meningitis in Olmsted County, Minnesota, from 1950 to 1981 was 10.9 per 100,000 population (Nicolosi et al., 1986). The incidence was markedly higher in children under age 1 year (82.4 per 100,000) and slightly higher in children between 1 and 4 years old (16.2 per 100,000). No deaths occurred in any of the 183 individuals with aseptic meningitis.