The technical difficulty of producing a vaccine and delivery (storage) requirements both affect vaccine cost. The committee based predictions in these areas on the nature of each vaccine candidate and on requirements for existing similar vaccines. Production technology and delivery requirements also affect the cost per dose, which, in turn, may affect vaccine acceptance.

The number of doses necessary to achieve the predicted protective efficacy of a vaccine and the route of administration also may affect vaccine acceptance. Predictions on these characteristics are based largely on the nature (including the probable antigenicity) of each vaccine candidate.

The predictions in Table 5.1 resulted from extensive deliberations by the full committee on estimates made by a subgroup, with suggested revisions from a broad spectrum of outside consultants. They were designed to reflect relative differences in the prospects of development of vaccine candidates; they are not expected to be precise descriptions of future events. Efforts to make predictions about vaccine development are complicated by many factors, including the rapid pace of new advances in biotechnology.

Although the outcome of scientific investigations cannot be predicted, the committee believes that the estimates and probabilities presented in Table 5.1 are reasonable, because the investigations on which they are based lie more in the realm of development than basic research. The factors considered in arriving at each prediction have been stated in as much detail as possible, in the belief that regular reappraisal of these factors is essential. The flexible nature of the model described in this report makes it easy to substitute alternative or updated predictions as they become available.