other factors that cannot be predicted with certainty. Several other nonquantifiable issues, all of which fall into the realm of the policymaker, also must be incorporated into the final judgment on vaccine priorities. These include:

• the goals of the responsible agency and its schedule for achieving them

• the ethical questions that must be considered in the distribution of benefits

• the most appropriate points at which the agency can exert influence and the opportunity and need for such influence

• the desired balance of the development portfolio

• the argument that can be made for treating certain vaccine development projects as unique because of their potential impact on immunization in general.

The committee sought to develop a flexible system that could be updated as necessary. This required an effort to explicitly identify all assumptions, estimates, and predictions incorporated in each calculation. Values incorporated into the calculations represent the committee’s best efforts to develop the necessary information. It is recognized that scientific opinion differs on some of the judgments and uncertainty surrounds other factors, e.g., disease incidence and efficacy data. The final format allows users of the system to perform sensitivity analyses, in which an estimate or prediction in a specific area, such as the probability of success, can be varied systematically across its plausible range to examine its impact on the final result. Some sensitivity analyses are discussed in Chapter 9.

Chapter 3 presents an overview of the approach used in this report. It also identifies certain basic assumptions that are maintained throughout the study. For example, in assessing the economic impact of vaccines, the report considers only direct costs—the cost of treatment for illness resulting from a disease, the cost of vaccine development, and the cost of vaccination programs. Effects of morbidity and mortality are expressed in non-monetary terms.

The committee defined vaccine candidates for accelerated development as those for which success was reasonably foreseeable within the next decade. The criterion for inclusion was whether a reasonable consensus could be identified on the nature of potential vaccine components (protective antigens). A more detailed description of the selection process appears in Appendix B.

The diseases and vaccine candidates chosen for the ranking process are shown in Table 1.1. Detailed information about individual candidates is presented in appendixes C through P. Some marginal candidates were excluded because the committee decided that it would be more appropriate to consider them in its deliberations on vaccine candidates for technologically less developed countries or because of resource