The expected health benefits to be derived from the development of a particular vaccine depend not only on the number of potential recipients of the vaccine (i.e., the target population), but also on the proportion of the target population who will actually receive it. This chapter describes the method used to arrive at an estimate of this proportion for each vaccine candidate.

To predict in advance the extent to which a vaccine candidate would be utilized is a difficult task, because vaccine utilization depends on many interrelated and complex factors. These factors include the following:

• vaccine availability

• the effects of statutory interventions such as school or day-care immunization requirements

• characteristics of the target population, including its access to health care providers and the ease with which its members can be identified by the health care system (in turn, dependent on size, composition, age, and socioeconomic status of the target population)

• characteristics of the vaccine affecting provider utilization (route of administration, storage conditions, shelf-life, cost of delivery, and special procedures required prior to or during vaccination)

• characteristics of the vaccine affecting patient acceptance (number of doses, route of administration, and cost)

• provider attitudes toward the vaccine, which are affected by perceptions of efficacy, safety, liability, ease of administration, profitability, professional consensus, and patient need or demand

• target population attitudes toward the vaccine, which are influenced by perceptions of the likelihood of contracting the disease, its severity if contracted, and the vaccine’s efficacy and safety.

These factors affect each other in ways that are not fully understood or quantifiable. To develop a method of predicting the likely utilization of new vaccines, the committee chose to consider three distinct groups of factors, namely, those influencing vaccine availability, lay attitudes, and provider attitudes.