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One of the most important qualitative results from this analysis was that measuring effectiveness and cost-effectiveness in terms of the reduction in prevalence of infection gives conclusions that conflict with assessment in terms of reduction in intensity. This is an important observation, given that most control programs are evaluated in terms only of the reduction in prevalence of infection. Figure 2.5 illustrates the relationship between cost and effectiveness for the four treatment options in the high- and low-transmission areas, in terms of reduction in infection and in heavy infection. Reduction in heavy infection was maximized in the hightransmission area and when treating frequently (every 4 months). Maximal reduction in infection prevalence, in contrast, was observed in the low-transmission area. Since, in this model, the costs are independent of endemicity, the cost-effectiveness ratios for heavy infection reduction are consistently lower in the high-transmission area than in the low-transmission area (see Table 2.4), suggesting it is more cost-effective to intervene in the high-transmission area. The cost-effectiveness ratios also indicate that the most cost-effective intervention in terms of heavy infection reduction is to treat every 2 years.
Although treating every 2 years minimizes the cost per heavy infection case prevented per person, treating every year provides an extra
F'g 2-5 The relationship between effectiveness (average infection and heavy infection cases prevented per person) and costs per capita at increasing frequency of treatment directed at Ascaris lumbricoides in a high- and low-transmission area. Modified from Guyatt et al. (1993), with permission
Table 2.4 Cost-effectiveness ratios and incremental cost-effectiveness ratios for delivering anthelminthic treatment directed at Ascaris lumbricoides at different frequencies in a high- and low-transmission area
High-transmission area Low-transmission area
Heavy infection Infection Heavy infection Infection
Cost per case prevented
2 yearly treatment 2.82 5.65 27.88 1.30
Yearly treatment 3.05 5.44 40.97 1.18
6 monthly treatment 3.48 4.34 72.01 1.21
4 monthly treatment 4.23 3.30 104.72 1.69
Extra cost per case prevented
1 years instead of 2 years 0.83 5.15 138.62 0.95
6 months vs. 1 year 4.19 3.50 722.38 1.25
4 months vs. 6 months 8.02 2.16 17 000 12.71
Modified from Guyatt et al. (1993), with permission.
gain in effectiveness, but at an extra cost. The extra cost required to obtain an extra unit gain in effectiveness, by treating more frequently, is expressed in terms of incremental cost-effectiveness ratios (see Table 2.4). In pictorial terms, these values can be understood by examining the gradient of a line joining any two alternative strategies (see Figure 2.10). The steeper the line, the more efficient the more costly alternative, as any increase in cost returns a high increase in
effectiveness. For instance, treating every year rather than every 2 years requires an extra cost of US$0.83 per extra gain in heavy infection cases prevented per person in the high-transmission area, and an extra US$138.62 in the low-transmission area (see Table 2.4). It is evident that treating more frequently requires less cost input to achieve a gain in disease prevalence reduction in a high-transmission area than in a low-transmission area. The incremental cost-
PRINCIPLES AND PRACTICE OF CLINICAL PARASITOLOGY
effectiveness ratios for disease reduction also reveal diminishing marginal returns, that is, as the frequency of treatment is increased, a higher cost investment is required per extra gain in effectiveness (for heavy infection reduction) (see Table 2.4). Studies have suggested that the interval between treatments for A. lumbricoides infection should be relatively short, of the order of 3-6 months (Cabrera et al., 1976; Arfaa and Ghadirian, 1977), depending on the infection rate of the endemic area (Morishita, 1972). These estimates, however, were based on the rate of rebound in infection prevalence after treatment. Previous studies have shown that prevalence recovers more rapidly than intensity and thus that the frequency of treatment required to maintain low levels of intensity is typically much less than that required to minimize prevalence (Anderson and Medley, 1985). The analyses by Guyatt et al. (1993), using the dynamic modelling approach, indicate further that relatively long intervals between treatment offers a cost-effective approach to morbidity reduction, and that measuring effectiveness in terms of infection prevalence reduction can lead to the identification of options that do not optimize morbidity control.
The Effect of Population Age Structure on the Costs of Control
The cost-effectiveness of age-targeted treatment has also been investigated for different delivery options using an age-structured dynamic model (Chan et al., 1994), which incorporates community demography parameters into parasite population dynamics (Anderson and May,