Recently, several population-based studies of the incidence of GBS applied strict criteria for the diagnosis of GBS, and relatively full case ascertainment appears to have been achieved (Hankey, 1987; Roman et al., unpublished observations, 1993; Winner and Evans, 1990). All of these studies indicate an incidence of GBS in the preschool age group of between 1.0 and 1.5 cases per 100,000 children per year, which is similar to the expected incidence in adults. The annual incidence of GBS in children in these studies is on the order of 0.1 cases per 100,000 children between the ages of 5 and 14 years (Winner and Evans, 1990) and 0.62 per 100,000 children and teenagers between the ages of 10 and 19 years (Hankey, 1987).
To obtain an idea of whether excess cases of GBS in relation to childhood vaccination occur each year, the following paradigm might be considered. Approximately 3,000 cases of GBS occur in the United States each year. If the preschool-age cohort makes up about 9 percent of the population, that would account for 270 cases per year if the incidence rate was uniform. As indicated above, the incidence of GBS in preschool children may well approximate the overall expected incidence in adults. Using some other assumptions, one can arrive at an estimate that about 6 percent of preschool-age children are within 5 days to 6 weeks of their most recent vaccination. If this is true, then one would expect about 16 cases of GBS per year in recently vaccinated preschool-age children. It is uncertain how many of these cases of GBS would be by chance alone and would be unrelated to vaccination. Nevertheless, excess cases of GBS occurring 5 days to 6 weeks after vaccination of preschool-age children have not been noted in the population-based studies mentioned above. This issue was also considered above in the section Antecedent Events. The data from the Monitoring System for Adverse Events Following Immunization show fewer cases of GBS per year, but such an analysis does not allow for the systematic underreporting that probably occurred.
In evaluating the vaccines considered in this report for a causal relation with demyelinating disease, several facts need to be considered:
Natural infections with measles and mumps viruses have been associated with ADEM.
ADEM and GBS in humans, similar to EAE or EAN in experimental animals, generally occur after an interval of 5 days to 6 weeks following infection (not clinical disease) or injection of antigen.
ADEM and GBS can occur after the administration of either live attenuated or killed vaccines (in the case of vaccinia virus and the swine influenza vaccines, respectively).