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.)