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estimates were to become available, however, they could readily be incorporated into the model.

The committee also excluded from the analysis the possible impact of vaccine use on the cost of current public health services such as disease surveillance or contact-tracing programs. A reduction in the number of cases of a single condition may not translate into a direct proportional change in the cost of public health services, which may be used in conjunction with a variety of other conditions as well. For example, a vaccine that prevents one type of STD will tend to reduce the burden on some services, but those services will continue to be needed in connection with other STDs.

It should also be noted that the committee’s analysis does not include as a cost factor a monetary value for changes in income associated with time lost to illness or use of a vaccine. This feature of the model reflects a widely accepted assumption in cost-effectiveness analysis (see Gold et al., 1996) that this opportunity cost of illness, in terms of both lost wages and time lost from unpaid work or leisure, is captured by the quality adjustment weights assigned to periods with and without illness. Thus, the cost of lost work is accounted for in nonmonetary terms rather than being excluded from the analysis.

This analysis could also include the impacts of the possible adverse effects of a vaccine, but the committee made an explicit decision not to incorporate this component. Adverse effects would generally result in a reduction in the health benefits produced by vaccine use and an increase in the costs of care. They could also limit the public’s acceptance of a vaccine. Estimating the magnitudes of these factors for each vaccine candidate would require assessments of the frequency of adverse effects, their nature and severity, the kinds of care required, and public reaction to them. The committee agreed that making meaningful predictions regarding any aspect of possible adverse effects of future vaccines would be very problematic and that there was no basis for distinguishing differences among the vaccines included in the study.

There is a reasonable basis for concluding that exclusion of adverse effects from the analysis has not altered the results in any meaningful way. Evidence regarding existing vaccines (IOM, 1991, 1994a, 1996a) suggests that adverse effects are infrequent and that very few are severe. This is consistent with preliminary analyses performed for the 1985 IOM report on the development of new vaccines, which found that estimated numbers of adverse effects produced minimal changes in the measures of disease burden and cost and did not alter the relative rankings of candidate vaccines. That committee also decided not to include estimates of adverse effects in the final analyses for its report. Although the present committee chose not to incorporate an estimate of adverse effects in its quantitative analysis, the issue of vaccine safety is of serious concern and is discussed further in several publications by other committees and forums held by the IOM (IOM, 1991; 1994a,b; 1996a; 1997a,b).

It is likely that some of the vaccines considered by the committee will become components of combination vaccine products similar to the familiar DTP products currently in general use. For vaccines intended for universal use, espe-



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