The logistic formulation ensures that the use rate will fall between 0.0 and 1.0 (i.e., between 0 and 100 percent).

The scores for pneumococcal, influenza, and pertussis vaccines and their voluntary use rates were used to obtain the following regression equations:

The anticipated use rate for each new vaccine was obtained by substituting its score (Table 6.4) into the appropriate regression equation, and solving for the use rate, p. The results are shown in Table 6.7.

The possibility of deriving anticipated use rates for new vaccines by adjusting the observed use rate of an existing comparable vaccine was evaluated. For example, the current use rate for MMR or DPT could be adjusted and applied to new pediatric vaccines. This approach was considered less satisfactory than the HBM for several reasons. Factors considered in adjusting the observed rate and their weights would be identified less explicitly, making assessment of changes in them more difficult. In addition, for certain new vaccines appropriate comparable vaccines do not exist. Although this approach was not adopted, it could be used to double check the calculated HBM anticipated use rates of selected new vaccines that do resemble existing vaccines.

Application of the system to recently developed vaccines, such as the mumps vaccine, may reveal possible limitations of the model as a predictive tool. For example, unexpected controversy may depress use rates, while unforeseeable events (e.g., the opportunity to combine the mumps vaccine with measles/rubella vaccine) may increase them.