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