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Because Beghi and colleagues (1984) found that the incidence of encephalopathy was higher in the first year of life than in the second or third years, these data could not be combined without taking into account the differences in the ages of the children. Assuming that the ratio of incidence in the first year life to the incidence in the second and third years as estimated in the study by Beghi and colleagues (22/15.8, or 1.4) is correct, each year of observation beyond the first birthday is equivalent to 1/1.4, or approximately 0.7 years of experience before the first birthday. By using this figure, adjusted background incidence rates were calculated for each study in Table 4-4 and for the four studies together by dividing the number of cases recorded outside of the immediate postvaccination interval by the number of first-year-equivalent years of observation. By pooling the results of the four studies in Table 4-4, the estimated background incidence rate for encephalopathy is estimated to be 78 per million children per year, or 0.43 per million children per 2-day period.

By comparing the estimated total incidence in the 2 days postvaccination derived from all eight studies listed in Table D-2 with the estimated background incidence rate during this same period, the RR in the 2 days postvaccination can be estimated at 7.6 per million divided by 0.43 per million = 17.7. The attributable risk for encephalopathy is the difference between the total incidence and the background incidence: 7.6 per million - 0.43 per million = 7.2 per million. Assuming that children, on average, receive three immunizations, the estimated attributable risk of encephalopathy is 2.4 per million immunizations.

If the studies of Pollock and Morris (1983) and Strom (1967), which relied on spontaneous reports for ascertainment, are excluded, the RR estimate is 17.1 and the attributable risk estimate is 2.3 per million immunizations. Relying only on the data in controlled studies of well-defined populations (Gale et al., 1990; Griffin et al. 1990; Walker et al., 1988), the total incidence in the week following vaccination is 2 cases per 173,771 children = 11.5 cases per million vaccinated children. Using a background rate of 0.43 x 7/2 per million = 1.5 per million, the RR estimate is 7.6 and the attributable risk estimate is 3.3 per million immunizations.

REFERENCES

Baraff LJ, Ablon WJ, Weiss RC. 1983. Possible temporal association between diphtheria-tetanus toxoid-pertussis vaccination and sudden infant death syndrome. Pediatric Infectious Diseases 2:7-11.

Beghi E, Nicolosi AN, Kurland LT, Mulder DW, Hauser WA, Shuster L. 1984. Encephalitis and aseptic meningitis, Olmsted County, Minnesota, 1950-1981. I. Epidemiology. Annals of Neurology 16:283-294.

Bernier RH, Frank JA, Dondero TJ, Turner P. 1982. Diphtheria-tetanus toxoids-pertussis vaccination and sudden infant deaths in Tennessee. Journal of Pediatrics 101:419-421.

Cody CL, Baraff LJ, Cherry JD, Marcy SM, Manclark CR. 1981. Nature and rates of adverse