BOX 4–1 Illustrating the Calculation of a Vaccine’s Health Benefits
The basic features of the calculation of QALYs can be illustrated with a simple scenario. Assume 100,000 cases of an illness X, occurring at an equal rate at all ages and no deaths. Half of the cases of disease result in a mild illness determined to have an HUl-based quality-adjustment weight of .90 and half in a moderate illness with a quality adjustment weight of .70. Either form of illness is assumed to last 2 weeks (.0384 years).
The quality-adjustment weights for illness X must be adjusted for the underlying health status of the population. Using survey-based data on general health status, the average quality-adjustment weight for the health status of the population without this illness is .896. Thus the adjustment weight for the mild form of illness becomes .806 (.90 • .896) and the weight for the moderate form of illness becomes .627 (.70 • .896).
To calculate QALYs, these adjustment weights are multiplied by the time spent with the illness. With a 2-week duration, a case of mild illness occurring in a given year accounts for .031 QALYs (.806 • .0384). With the same 2-week duration, a case of moderate illness accounts for .024 QALYs (.627• .0384). For an individual in the general population not experiencing this illness, the same 2-week period would represent .034 QALYs (.896 • .0384).
Use of a vaccine that prevents illness X would result in a gain of .003 QALYs for a case of mild illness (.034–.031) and .010 QALYs for a case of moderate illness (.034–.024). With cases distributed evenly between mild and moderate illness, the average gain would be .007 QALYs [(.5 • .003) + (.5 • .010)]. With 100,000 cases per year, the annual gain for the population would amount to 700 QALYs (.007 • 100,000). (The complete analysis would also require discounting QALYs for the interval between age at vaccination and average age of onset of illness X.)
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, and these quality-adjusted time periods are added together. In theory, “perfect health” carries a weight of 1.0, giving full value to periods to which it applies. Death carries a weight of 0.0. A health state judged to be equivalent in quality to death would also have a weight of 0.0, meaning that time spent in that health state would have a QALY value of 0.0. A condition considered worse than death can be assigned a negative weight. These quality-adjusted periods can be summed over a person’s expected lifetime (or some other specified period of time).