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Vaccines for the 21st Century: A Tool for Decisionmaking
parallel calculation in which the population baseline quality-adjustment weight was multiplied by the duration of the health state determined the QALYs that the general population would experience without the condition under study. Both sets of QALYs were then discounted to adjust for the time from average age at vaccination to average age at onset. Calculation of QALYs when the condition under study is present can be represented as
where qi is the QALY value for health state i, wi is the baseline-adjusted quality weight for the health state, ti is the duration of the health state, r is the discount rate, To is the time from average age at vaccination to average age at onset, and tm is the duration of health states that intervene between the onset of the condition and health state i. If related health states that persist for at least 1 year intervene between the onset of the condition and health state i, the discount period must be increased by tm. To calculate q'i, the corresponding QALY value for the general population, wi is replaced by w', the population baseline quality-adjustment weight.
Calculation of QALYs for two kinds of health states required alternative approaches. First, for states that persist for a specified multiyear period rather than a period of days or weeks, allowance must be made for discounting the stream of QALYs associated with the health state. In these health states, QALYs discounted to the beginning of the health state were calculated by using the Excel present-value function. This value is then further discounted for the period To+tm.
Second, for health states that persist for the normal remaining lifetime, the discounted quality-adjusted life expectancy at onset (eo*) was used as the value of the QALYs that the general population would experience without the condition under study. To estimate QALYs with the condition, eo* was multiplied by the committee’s quality-adjustment weight for the health state hi (Adjustment for the baseline health status of the population is already reflected in the quality-adjusted life expectancy.) Additional discounting for the period from vaccination to onset (To+tm) is performed.
Mortality The impact of mortality on QALYs was estimated as described above for morbidity associated with lifetime health states. The death rates obtained for each condition reflect deaths that occur within a short time following onset of the condition. The discounted quality-adjusted life expectancy at the average age at death (ed*) provides the estimate of QALYs lost due to deaths, which must be discounted for the period To+Td, where To is the time from average age at vaccination to average age at onset and Td is the difference between average age at onset of illness and average age at illness-related death. If the age pattern of deaths is similar to the age pattern of illness, Td=0. If deaths are more common among younger or older patients, Td≠0.