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Vaccines for the 21st Century: A Tool for Decisionmaking
(Microsoft Corporation, 1984–1994), operating on a personal computer. Using the basic template, a separate file was created for each case. The following data were entered: age-specific incidence and death rates, average age at immunization, morbidity scenarios and associated quality-adjustment weights, typical health care provided for the condition and its associated costs, size of the target population, number of vaccine doses required, cost per dose, estimated vaccine efficacy, anticipated steady-state vaccine utilization rates among the target population, remaining development costs, expected time until licensure, and time from licensure until anticipated utilization rates are reached. The basic template was modified to accommodate variations among the cases in the patterns of illness (e.g., distinctive characteristics by age or sex), in the features of the morbidity scenarios (e.g., numbers of health states included and mix of acute and chronic health states), and in the types of care required. Detailed descriptions of the data and calculations used for each case are provided in the Appendixes.
CALCULATION OF HEALTH BENEFITS
Health benefits were measured using quality-adjusted life years (QALYs). Described below is the multistep process followed by the committee to estimate vaccine-related health benefits and calculate QALYs.
Quality-Adjusted Life Years
QALYs reflect the combined impact of morbidity and mortality on the health-related quality of years of life lived. To calculate QALYs, a quality-adjustment weight is applied to each period of time during which a person experiences a changed health state due to a particular condition. These quality-adjusted periods can be summed over a person’s expected lifetime (or some other specified period of time). This can be illustrated in a simplified form as
where Q is the total QALYs experienced by the individual, W1 is the quality-adjustment weight associated with health state 1, and t1 is the amount of time spent in that health state. Thus, the lifetime QALYs for an individual who lives for 70 years in perfect health, experiences six months of impaired health from condition 1, and dies five years sooner than the average life expectancy can be represented as