The illness that could be averted by an improved pertussis vaccine is considered to be equal to the burden of adverse side effects arising from the current vaccination program. This is calculated from incidence rates for adverse effects determined by Hinman and Koplan (1984), assuming that 95 percent of the projected 1984 birth cohort receive the recommended five doses (three in the first year of life and the remaining two before the age of five). Hence, the annual number of doses is 3,788,337×0.95×5=17,994,600; and episodes are distributed between the under one year and the 1–4 years age groups in a ratio of 3:2.
Minor reactions (Morbidity Category A illness) occur at a rate/dose of 1:2.5; hence, a total of 7,197,840 episodes are expected. These are assumed to last two days.
Reactions falling into Morbidity Category B occur at rates of 1:1,750 for both convulsions and collapse, and 1:926 for high-pitched crying. These result in estimates of 10,283 cases of convulsion, 10,283 cases of collapse, and 19,433 cases of high-pitched crying per year. Thus, the total in Category B is 39,999 episodes, each assumed to last two days.
Hospitalizations (for two days) are estimated to result from 25 percent of physician contacts for convulsions and collapse (which are estimated to be half of the total number of convulsions and collapse), i.e., 0.25×2×10,283×0.5=2,570 cases. Hospitalizations (for approximately two days) are estimated to result from 10 percent of the cases with unusual crying that seek medical care (estimated to be 25 percent of all cases); i.e., 0.1×0.25×19,433=485 cases. Encephalitis occurs at a per dose rate of 1:110,000 and results in 164 cases requiring hospitalization for an estimated 15 days. The total number of hospitalizations is therefore 3,219 cases distributed between the under 1 and 1–4 years age groups in a ratio of 3:2 (doses under one year:doses over one year, i.e., 1,931:1,288). The average weighted duration of these hospitalizations is three days.
Chronic sequelae from encephalitis occur at a per dose rate of 1:310,000, resulting in a total of 58 cases. These have been distributed among Morbidity Categories D, E, and F, and in a ratio of 3:2 between the under 1 year and the 1–4 years age groups.
Table C.1 summarizes the burden of illness arising from adverse vaccine reactions that could be eliminated with an improved pertussis vaccine. This burden is potentially preventable if an improved vaccine with equal efficacy could be substituted for the existing vaccine.
The concept of “infant mortality equivalence” is used to standardize disease burden values (see Chapter 4). Vaccine preventable illness values for pertussis reflect the total elimination of side effects caused by the current vaccine. They are calculated using the estimates in Table C.1 and the two sets of IME values used throughout this