The preceding analysis of life years gained takes into account the mortality effect of HIV and HCV on persons within the target population, namely injecting drug users. However, this analysis does not take into account any differences in the quality of life for those with HIV or HCV compared to those without the disease. As discussed in Section 5.2, the application of an adjustment factor to take account of the quality of life effects of these diseases leads to a measure referred to as Quality Adjusted Life Years (QALYs). Comparing this measure to the number of life years that the affected population lives in a disease-free state (i.e. by avoiding HIV and HCV) provides a measure of the QALYs gained as a result of NSPs. QALYs gained therefore incorporate both the quantity of life gained, and the quality of life gained by avoiding HIV and HCV.
The outcomes of this analysis are presented in Table 5.5.1 (See Appendix E), and in Figure 5.4, Figure 5.5, Figure 5.6 and Figure 5.7.
In Figure 5.4 and Figure 5.5 the curves "HIV/HCV QALYs Gained" represent the quality adjusted life years for persons who would have had HIV/HCV, but for the effect of NSPs. The gap between these curves and the curves "Life Years Gained" represents the quality effect of HIV and HCV on their lives. The 25,000 persons avoiding HIV are expected to gain an additional 715,000 quality adjusted life years than if they had contracted the disease. In comparison, the 21,000 persons avoiding HCV are expected to gain about 120,000 quality adjusted life years over their lifetime. The difference between the two groups is largely attributable to the greater effect of HIV on the "quantity" of life compared to HCV, rather than the "quality" effect.
In the case of HIV, the number of life years gained each year increases up to the year 2033, and thereafter continues at a progressively slower rate. The curve for QALYs gained generally follows a similar pattern, reflecting the dominant effect of the "quantity" component.
In contrast, the number of life years gained for persons avoiding HCV is relatively small. However, when considering the effect of HCV on the quality of life, considerable gains are evident. These gains are relatively constant up to the year 2035, then decline each year to death.
Figure 5.6 and Figure 5.7 illustrate the cumulative number of life years and QALYs gained by avoiding HIV and HCV. The shape of the curves "Life Years Gained" illustrates the progressive effect of the different mortality rates and is considerably steeper for HIV than for HCV. The increasing gap between "Life Years Gained" and "QALYs Gained", and the timing of its emergence, illustrates the differences in quality of life effect between the two diseases.
As previously noted, it is not uncommon to discount QALYs gained in the future in the same way aw we have discounted future financial benefits. This approach is based on the principle that an improvement in the quality of life is likely to be valued more if it occurs earlier than if it occurs later in life, just as a dollar gained earlier is likely to be valued more than a dollar gained later.
Applying the same discount rates used in the financial analysis (viz 5%, 3% and 0%) to QALYs gained results in the figures shown in Table 5.6.1
A total of approximately 715,000 QALYs were gained by the avoidance of HIV, the present value of which, at a discount rate of 5%, is 138,000 QALYs (248,000 at a 3% discount rate). The equivalent gains for HCV are 120,000 QALYs over their lifetime, the present value of which (discounted at 5%) is 32,000 QALYs (50,000 at 3%). Discounting has the effect of reducing the present value of gains made in later years relative to those made in earlier years. Consequently, the ratio of the present value of QALYs gained to total QALYs gained for HIV (i.e. 138,072/715,245) is lower than that for HCV (i.e. 32,207/119,992), reflecting the fact that HCV makes a higher proportion of its QALY gains in the earlier years compared to HIV.
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Figure 5.4 Life Years and QALYs gained by HIV survivors
Text version of Figure 5.4Figures in this description are approximate as they have been read from the graph.
Figure 5.4 consists of two graphs with similar shapes.
Life years gained by HIV survivors increases dramatically from 100 in 1991 to 15,800 in 2033, before decreasing sharply to 1,000 by 2063.
HIV QALYs gained by HIV survivors increases dramatically from 100 in 1991 to 16,800 in 2031, before decreasing sharply to 1,000 by 2063.
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Figure 5.5 Life Years and QALYs gained by HCV survivors
Text version of Figure 5.5Figures in this description are approximate as they have been read from the graph.
Figure 5.5 consists of two graphs.
Life years gained by HCV survivors remains relatively stable at less than 100 for all years between 1991 and 2072.
HIV QALYs gained by HCV survivors increases sharply from 700 in 1991 to 2,200 in 2000, remaining relatively stable until 2036, where it decreases sharply to less than 100 by 2072.
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Figure 5.6 Cumulative Life Years and QALYs gained by HIV survivors
Text version of Figure 5.6Figures in this description are approximate as they have been read from the graph.
Figure 5.6 consists of two graphs with similar shapes.
Cumulative life years increases slowly from zero in 1991 to about 50,000 in 2003, before steadily increasing to about 650,000 in 2051 and remaining steady at this level to 2070.
Cumulative QALYs increases slowly from zero in 1991 to about 50,000 in 2012, before steadily increasing to about 550,000 in 2051. It then slowly rises to about 600,000 in 2063 and then remains steady at this level to 2070.
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Figure 5.7 Cumulative Life Years and QALYs gained by HCV survivors
Text version of Figure 5.7Figures in this description are approximate as they have been read from the graph.
Figure 5.7 consists of two graphs.
Cumulative life years gained remain relatively constant at about zero life years from 1991 to 2075.
Cumulative QALYs gained increases steadily from about zero in 1991 to about 120,000 in 2063, and then remains constant at that level for subsequent years.
Table 5.6.1 Net present value (1991) of QALYs gained for HIV and HCV
|Discount rate||NPV of QALYs gained for HIV||NPV of QALYs gained for HCV||NPV of QALYs gained for HIV and HCV|