report. Using IME values based on a median of committee member perspectives, the vaccine preventable illness value for an improved pertussis vaccine is 62; with the age-neutral perspective, the value is 60.
The basis for calculating the reduction in morbidity and mortality that could be achieved with an improved pertussis vaccine is different from that used to calculate PRMM for totally new vaccines, and is described above.
The improved vaccine is anticipated to have the same efficacy and target population as the existing vaccine, so the maximum possible reduction in morbidity and mortality is the elimination of adverse effects—as expressed in the VPI values above.
The PRMM calculation does not recognize, however, that the new vaccine also will cause some adverse reactions (Table 5.1). To determine how these side effects will affect the benefits from the new vaccine, it is necessary to estimate the number of episodes in each age group and then adjust them using the appropriate infant mortality equivalence values. Five to 10 percent of vaccinees are expected to have mild local reactions (which are considered less significant than illness in Morbidity Category A) and 0.1 percent are expected to have moderate systemic reactions that will fall into Morbidity Category B and last for two days. Applying these percentages to the estimated number of doses (assuming 95 percent of the predicted 1984 birth cohort of 3,788,337 receives five doses, three in the first year of life and two more between one and five years of age), gives a total of 3,788,337 ×0.95×3×0.001=10,797 adverse episodes in infants under one year of age. In the 1–4 years age group, the total is 3,788,337×0.95×2 ×0.001=7,198.
Assuming a two-day duration and using appropriate IME values, the adjusted figure for adverse effects of the improved vaccine using the committee median perspective would be
Using the age-neutral perspective, it would be
These values do not significantly change the net potential benefit values for the new pertussis vaccine. They remain 62 and 60 using the committee median and age neutral perspectives, respectively.
Use of these values for comparing vaccines is described in Chapter 7.