For some conditions, deaths occur following a period of chronic impairment. These have been assumed to be additional condition-related deaths beyond those reflected in the age-specific death rates. The age at death is calculated as the average age at onset of the condition plus the duration of health states experienced between onset and death. The discounted quality-adjusted life expectancy at the age at death is used to calculate the QALYs lost due to these deaths. Additional discounting for the period To plus the interval between onset and death must be included. Life expectancy at the time of these deaths was estimated by interpolation from the values calculated for specific ages of onset of illness (see above).
The QALYs associated with each health state were combined. For each morbidity scenario, the state-specific QALYs were summed, and the QALYs lived with the condition under study were subtracted from the QALYs for the general population without the condition. This provides an estimate of the QALYs that could be gained in each scenario with vaccine use.
The scenario-specific QALYs to be gained were multiplied by the proportion of cases of illness experiencing that scenario and were summed across all scenarios. This total was multiplied by the number of cases, or by the number of deaths for the QALYs associated with mortality, to calculate the overall benefit that the use of a vaccine for the condition under study would be expected to have in the population. If subpopulations were used in the analysis, the results were calculated for each subpopulation and the subpopulation results were summed to produce an estimate of the total health benefit.
Q;s=[∑j (qj • Pj)] • Ns, (10)
where Q;s is the total QALYs to be gained with vaccine use across all subpopulations, qj the individual QALY gain in scenario j; Pj is the proportion of cases of illness experiencing scenario j, and Ns is the total number of cases of illness in subpopulations.
As shown in Equation 1, the cost components of the numerator of the cost-effectiveness ratio include the costs of vaccine development (CD), the cost of vaccine use (CI), and the reduction in health care and related costs (CC) that would be expected with vaccine use. All cost estimates are presented in constant dollars.