Cost-effectiveness of blindness prevention by the Onchocerciasis Control Programme in Upper Volta*

Bulletin ofthe World Health Organization, 62 (5): 795- 802 (1984) (3 World Health Organization 1984 Cost-effectiveness of blindness prevention by the Onchocerciasis Control Programme in Upper Volta* A. PROST & N. PRESCOTT The article presents a cost-effectiveness analysis of the Onchocerciasis Control Pro- gramme in Upper Volta. The analysis uses a new approach to the measurement of health project effectiveness, by considering the number of healthy years of life added by the prevention ofpermanent disability andpremature death attributable to onchocercal blind- ness. The approach emphasizes the central role of social value judgements in allocating health resources in particular the relative weights assigned to preventing disability and postponing death, present and future health benefits, and health gains among productive and non-productive individuals. The quantitative results yield the following cost- effectiveness estimatesfor blindness prevention through onchocerciasis control: US$20per year of healthy life and per productive year of healthy life added, and US$150 per discountedyear ofhealthy life andper discounted productiveyear ofhealthy life added. As an illustrative example, a comparison is made with estimates of the cost-effectiveness of measles immunization. The analysis proposed in this paper differs from previous economic analyses of onchocerciasis control (see 5)° in three important respects. First, it uses em- pirical data as the basis for an estimate of the epi- demiological effectiveness of the intervention. Second, it focuses on the prevention of permanent disability and premature death due to onchocercal blindness as the major health improvement attri- butable to onchocerciasis control. Third, it empha- sizes cost-effectiveness rather than cost-benefit analysis. This limitation is imposed by the practical difficulty of undertaking a comprehensive and empirically defensible assessment of all the benefits of onchocerciasis control. In particular, the extent to which control of partial visual impairment and infec- tion without ocular involvement would increase the effective supply of labour, and the extent to which control would increase the effective supply of land by inducing new settlement in the river valleys, have not been clearly established. The cost-effectiveness approach is limited because it foregoes the oppor- tunity provided by cost-benefit analysis to compare the relative desirability of investing in onchocerciasis control with alternative investments in other sectors. However, it does permit useful judgements to be made about the relative efficiency of allocating scarce * From the Population, Health and Nutrition Department, The World Bank, 1818 H Street, NW, Washington, DC 20433, USA. The views and interpretations in this paper are those of the authors and should not be attributed to the World Bank or its affiliated organiz- ations, or to any individual acting on their behalf. a PRESCOTT, N. M. The economics of malaria, filariasis, and human trypanosomiasis. Geneva, World Health Organization, 1980(unpublished document, TDR/SER (SC-1)/80.4). resources to onchocerciasis control compared with other possible investments within the health sector. As an illustrative example, a comparison is made with estimates of the cost-effectiveness of measles immu- nization. BACKGROUND A summary of data on the distribution and preva- lence of blindness in Upper Volta during the period 1970-75 was published recently, based on infor- mation collected by the fiscal authorities (6). The average prevalence of blindness in the whole country was estimated to be 7-9 per 1000 population. How- ever, in many districts, blindness affects 30-50 per 1000 population, and in selected villages blindness rates as high as 100-130 per 1000 population can be observed. Districts where blindness rates exceed 10 per 1000 are all located in areas that are hyperendemic for onchocerciasis. The 1975 census estimated the total population of these areas to be 800 000. Foci of onchocerciasis also exist outside these administrative units, but because of their limited size, do not greatly influence the overall average blindness rate in the area. Therefore, the population living in areas where the risk of blindness is significantly higher than the average rate in the country is estimated at approxi- mately 1 million people. This increased risk of blind- ness is associated with a greater intensity of infection with onchocerciasis. For the purpose of the present study, it is assumed that among the I million people at 4459 -795- 796 A. PROST & N. PRESCOTT Table 1. Summary of data on blindness among a sample of 50 000 people in the south-west of Upper Volta, 1960-80" Average annual incidence of blindness Mean age at onset Mean duration of life Normal life expectancy Endemicity (per 1000 population) of blindness (years) after onset (years) at age of onsetb (years) Hyperendemic 5 39 9 23 Mesoendemic 1.8 45 7 20 " According to Prost & Paris (7). " From reference 8. risk, 500 000 live in areas that are hyperendemic for onchocerciasis, and the remaining 500 000 in meso- endemic areas. This assumption is rather conserva- tive. In hyperendemic areas of Upper Volta, the inci- dence of blindness varies from 3 cases per 1000 popu- lation per year to 11 per 1000 (9) with an average rate of 5 per 1000 (7). The incidence ranges from 1.7 to 2 per 1000 in mesoendemic areas, with an average of 1.8, and decreases to 0.5 per 1000 in areas where onchocerciasis is no longer the leading cause of blind- ness (7). For the purpose of this study, we assume that the annual incidence of blindness due to causes other than onchocerciasis is about 1 per 1000. If the average lifespan in the blind is taken as 9 years, this figure for incidence is consistent with observations by Rollandb that the prevalence of blindness due to causes other than onchocerciasis is about 9 per 1000 in endemic zones. It is also consistent with indications from Onchocerciasis Control Programme surveys, that onchocerciasis is the cause of no more than 80%7o of blindness cases in hyperendemic areas. The average age at onset of blindness in hyper- endemic areas is 39 years. It is delayed to 49-51 years in areas of very low prevalence of onchocerciasis, with an intermediate figure of 45 years in meso- endemic areas (7). The mean duration of life in the blind ranges from 7 years in mesoendemic areas to 9 years in hyperendemic areas, 13-14 years shorter than the life expectancy of the non-blind (Table 1) (7). This observation is consistent with the results of a follow- up study of blind and sighted people in Upper Volta (8), which indicated that blindness reduces life expec- tancy by at least 13 years at age 30. It should be under- stood that the association of onchocercal blindness with excess mortality is only indirectly causal, re- flecting social rather than pathological consequences of blindness. h ROLLAND, A. Les cecit0s onchocerquiennes dans les zones couvertes par le projet regional de lutte contre l'onchocercose en Afrique de l'ouest. Bamako, Institut d'ophtalmologie tropicale de l'Afrique, 1972 (unpublished document). MEASUREMENT OF COSTS The measurement of costs is complicated by the problem of allocating joint costs between Upper Volta and the other six countries included in the Onchocerciasis Control Programme. The control of onchocerciasis in the Volta River basin began in 1975 with larviciding operations carried out in the west of Upper Volta. Control was progressively extended to other river basins to cover the entire programme area in seven west African countries by the end of 1977. At this stage, the total annual expenditure was US$10.2 million (Table 2). We will use this figure in the present analysis because the increase in the budget in later years was due to an extension of the programme area in Ivory Coast, the use of alternative and more expensive insecticides in southern regions, and other factors that are not relevant to Upper Volta. In 1977, the area covered by the programme in Upper Volta represented 32% of the total area; of the 510 000 km of rivers treated in 1977, an estimated 27% were in Upper Volta, and 250%o of operational staff were assigned to the country. Therefore, the cost of onchocerciasis control in Upper Volta represented about 25% of the total programme expenditure, i.e., US$2.6 million per year. At present, expenditure in Table 2. Expenditure on the Onchocerciasis Control Programme, 1975-81 Expenditure Year (US$) 1975 6133458 1976 10586071 1977 10167488 1978 12051 185 1979 14296696 1980 16506724 1981 16646363 COST-EFFECTIVENESS OF BLINDNESS PREVENTION BY THE OCP 797 Upper Volta is significantly lower, as a result of inter- ruption of larvicide spraying, the reduction of the surveillance network, and redeployment of staff. However, these measures were made possible by the achievement of successful control in the central zone of the programme and by ongoing operations in sur- rounding areas. Upper Volta is now protected by treatments carried out in neighbouring countries. Present expenditure in Upper Volta thus does not accurately represent the cost of maintaining control of onchocerciasis in the country. MEASUREMENT OF EFFECTIVENESS Since 1977, the Onchocerciasis Control Programme has achieved complete interruption of the transmission of onchocerciasis in Upper Volta.' The decrease in the prevalence of the disease has been gradual because people who were infected before vector control operations were started still carry living parasites. However, ophthalmological follow-up indicated that the incidence of onchocercal blindness in hyperendemic areas of Upper Volta decreased to about 2.5 per 1000 per year during the period 1975-78 (11), and that the overall incidence of blindness de- creased to 3.2 per 1000 per year during the period 1975-80.d Since 1980, only sporadic new cases of blindness have been recorded, resulting from the deterioration of old and irreversible eye lesions. A comparison of these data with the precontrol data presented above enables us to estimate changes in the annual incidence of blindness (Table 3). The precontrol figures given are the average rates found in a study in the south-west of the country (7). The incidence of blindness due to onchocerciasis since 1981 is considered to be negligible, and an incidence of 1 per 1000 is attributed to other causes. The figures given in Table 3 are based on the average incidence of 3.2 (all causes) and 2.5 (onchocerciasis alone) per 1000 recorded in the ophthalmological follow-up during the period 1975-80. The difference between the precontrol and post- control incidence of onchocercal blindness can be attributed to the intervention of the Onchocerciasis Control Programme. These data can be transformed into different measures of effectiveness, as follows. Cases of blindness prevented The simplest measure of effectiveness is the number ' Onchocerciasis Control Programme. Results ofepidemiological evaluation six years after the start of vector control operations(1975-1981).Geneva, World Health Organization, 1981 (unpub- lished document, OCP/EAC 2.3). d Ocular onchocerciasis, five years after commencing vector control in the Volta River Basin. Geneva, World Health Organiz- ation, 1981 (unpublished document, OCP/EAC 2.2). Table 3. Estimated incidence of blindness per 1000 population in Upper Volta, 1975-82 All causes Due to onchocerciasis Hyper- Meso- Hyper- Meso- endemic endemic endemic endemic areas areas areas areas Precontrol 5.0 1.8 4.0 0.8 1975 5.0 1.8 4.0 0.8 1976 4.2 1.7 3.2 0.7 1977 3.5 1.5 2.5 0.5 1978 2.8 1.3 1.8 0.3 1979 2.0 1.1 1.0 0.1 1980 1.3 1.0 0.3 0.0 1981 1.0 1.0 0.0 0.0 1982 1.0 1.0 0.0 0.0 Table 4. Estimated number of cases of onchocercal blindness prevented by the Onchocerciasis Control Programme (OCP) in Upper Volta, 1975-82 Predicted no. Estimated no. No. of cases Year of cases of cases prevented without OCP occurring by OCP 1975 2400 2400 0 1976 2441 1983 458 1977 2483 1552 931 1978 2524 1105 1420 1979 2568 589 1979 1980 2611 163 2448 1981 2656 0 2656 1982 2701 0 2701 of cases of blindness prevented by onchocerciasis control. This can be estimated from the blindness rates, before and after intervention, shown in Table 3 and projections of the size of the population at risk, assuming an annual growth rate of the population of 1.7%o. The resulting estimates of the number of cases of blindness prevented in each year from 1975 to 1982 are presented in Table 4. Although it is simple to estimate the number of cases of blindness prevented, this figure is of very limited value as a measure of effectiveness for use in cost-effectiveness analysis, since it only permits com- parisons with other health interventions that prevent blindness. For policy purposes, it would be more useful to consider a composite health status index that aggregates morbidity and mortality reductions into a single measure that is applicable in all situations. One such measure is the number of years of healthy life A. PROST & N. PRESCOTT added. A measure of this type was applied for the first time in a developing country by the Ghana Health Assessment Project Team to assess the combined effects of morbidity and mortality due to different diseases in Ghana (2). This measure represents a special case of the general class of health status index models originally devised for health planning purposes in developed countries (12). Years of healthy life added The number of years of healthy life added by pre- venting onchocercal blindness is estimated by aggre- gating the effects of preventing disability and postponing death. The number of years by which death is postponed is estimated to be the difference between normal life expectancy at the age of onset of blindness and the average age of death among the onchocercal blind. The relevant data were given in Table 1. These indicate that, in hyperendemic areas, blind people expect an average of 9 years of disability followed by death 14 years prematurely. The corres- ponding figures for mesoendemic areas are 7 and 13 years. Assuming that blindness results in complete disability, and that one year of complete disability is equivalent to one year of death, as was assumed by the Ghana Health Assessment Project Team (2), each case of blindness prevented is estimated to add 23 years of healthy life in hyperendemic areas and 20 years in mesoendemic areas. The resulting transform- ation from cases of blindness prevented into healthy years of life added is shown in Table 5. Our estimate of this measure of effectiveness does not include the additional health benefits of averting the periods of infection without ocular involvement and subsequent partial visual impairment, because the lack of adequate data precludes a realistic assessment of these effects. It is important to note that the number of years of healthy life added that are attributed to each year in Table 5. Undiscounted years of healthy life added by the Onchocerciasis Control Programme in Upper Volta, 1975-82 By preventing By postponing Year disability death Total 1975 0 0 0 1976 4 020 6 361 10 381 1977 8 069 12 879 20 948 1978 12 254 19 617 31 871 1979 17 063 27 332 44 395 1980 21 162 33 837 54 999 1981 23 018 36 741 59 759 1982 23 409 37 364 60 773 Table 5 do not actually occur in that year. Rather, the measure represents an estimate of the future number of years of healthy life that are added by preventing the onset of blindness in each year. It will be assumed in this paper that the prevention of a new case of blindness in a hyperendemic area in any year t will lead to the addition of one year of prevented disability in each of the subsequent years (t + 1) to (t + 9), followed by the addition of one year of prevented premature death in each of the years (t+ 10) to (t + 23). Similarly, prevention of the onset of blindness in mesoendemic areas in year t is assumed to add one year of prevented disability in each of the years (t + 1) to (t +7) and one year of prevented premature death in each of the years (t + 9) to (t + 20). For example, the health benefit produced by preventing one new case of blindness in a hyperendemic area in 1976 would begin to accrue one year later in 1977 and terminate 23 years later in 1999. The problem of weighting The number of years of healthy life added is pro- posed as an appropriate measure of effectiveness for use in cost-effectiveness analysis. However, it is ob- vious that the aggregation of reductions in morbidity and mortality into a single measure necessarily in- volves making value judgements about the relative weights that should be assigned to each component. We consider here three types of value judgement that are implicit in the formulation of the measure of years of healthy life added. Weighting disability versus death. The measure assumes that one year of complete disability is equi- valent to one year of premature death. The argument is that complete disability is economically equivalent to death, in the sense that the completely disabled person is totally non-productive. However, it could be argued that complete disability is worse than pre- mature death and should therefore be given greater weight. The principal reasoning behind this argument is that, in addition to foregoing the output of a blind worker, the other members of society are imposed with the burden of meeting his consumption require- ments. Consumption per capita is thus lower if the worker is completely disabled than it would be if he were dead. The extent to which this consideration should influence the relative weights given to dis- ability and death is difficult to resolve and is not attempted here. Instead, we retain the assumption of equal weights and simply note the view that a higher weight for disability may be more appropriate. Weighting for age preference. The measure assumes that additional years of life are equally valuable, regardless of the age at which they accrue. 798 COST-EFFECTIVENESS OF BLINDNESS PREVENTION BY THE OCP Thus, prevention of child mortality appears to be much more valuable than prevention of adult mortality, simply because life expectancy for a child is much greater than it is for an adult. This approach is consistent with the popular interest in primary health care and its emphasis on reducing infant and child mortality. However, it conflicts with the common notion that adult mortality is more serious than child mortality. For example, suppose that there is a choice between saving the life of a mother or her baby during delivery. There is no doubt that, in most cases, the physician's preference would be to save the mother. An extreme method of taking account of this view would be to assign zero weight to years of healthy life added in the non-productive age group and a weight of one to those added in the productive age group. For this purpose, it is convenient to use the conventional definition of the working age group as 15-60 years. We adopt this approach in a subsequent comparison of onchocerciasis control with measles immunization. Weighting for time preference. The measure assumes that a year of healthy life added is equally valuable regardless of when it accrues. Thus, for example, saving 100 lives 10 years in the future would be considered just as valuable as saving 100 lives this year. This is not consistent with an established con- vention in the economic analysis of projects that future benefits (and costs) should be assigned progres- sively lower weight the later they occur in the life of the project. An important justification for this convention is that there exists a clear social preference for receiving benefits sooner rather than later. The implication is that society would, in fact, prefer to save 100 lives this year rather than in 10 years' time. The relative value of benefits occurring at different times is expressed by a social discount rate. A fairly typical value of 10% will be used for illustrative purposes here. This implies that 100 years of healthy life added in a given year is worth the same as 110 years of healthy life added in the following year. Conversely, it implies that 100 years of healthy life added next year is worth the same as 91 years added this year. More generally, the discount factor applied to the quantity of benefits in each year t is: (i1 rY where r is the discount rate (expressed in decimals). Weighting the benefit accruing in any year t by the appropriate discount factor yields the present discounted value of those benefits in the pre-project year, t =0. Detailed tables of discount factors for different discount rates and time periods have been compiled (2). For a discussion and example of the application of discounting to the physical health benefits of tuberculosis control, see Feldstein et al. (1). The estimation of the number of discounted years of healthy life added can be explained as follows. Prevention of the onset of blindness in any year I results in the addition of one year of healthy life in each of the subsequent years (t + 1) to (t + n), where n equals 23 in hyperendemic and 20 in mesoendemic areas. The discounted value of this increment is the sum of the benefit of one year accruing in each of these years weighted by their appropriate discount factors. For hyperendemic areas, this sum equals 8.88 discounted years of healthy life added, of which 5.76 years is contributed by prevented disability and 3.12 years by prevented premature death. Similarly, prevention of a case of blindness in mesoendemic areas yields 8.51 disc9unted years of healthy life added, comprising 4.87 years added by prevention of disability and 3.64 years by prevention of premature death. Clearly, the discounted value of healthy years of life added by preventing blindness decreases, the further into the future the blindness is prevented. Thus, prevention of a case of blindness in a hyperendemic area in year 10 would yield only 3.43 discounted years of healthy life added, and prevention of a case in a mesoendemic area would add 3.28 discounted years. Estimates of the number of dis- counted years of healthy life added each year in the first 8 years of the Onchocerciasis Control Pro- gramme are given in Table 6. A comparison of the figures in Table 6 with the esti- mates of undiscounted years added (shown in Table 5) clearly demonstrates the effect of discounting. While the number of undiscounted years added continues to increase in each year of the project, the discounted value starts to decline after 1980. In 1982, the discounted value amounts to less than one-fifth of the undiscounted value generated by a reduction in the incidence of blindness in that year. Table 6. Discounted years of healthy life added by the Onchocerciasis Control Programme in Upper Volta, 1975-82 By preventing By postponing Year disability death Total 1975 0 0 0 1976 2 124 1 215 3 339 1977 3 924 2 246 6 170 1978 5 442 3 114 8 556 1979 6 894 3 946 10 840 1980 7 753 4 437 12 190 1981 7 647 4 376 12 023 1982 7070 4046 11 116 799 A. PROST & N. PRESCOTT COST-EFFECTIVENESS ANALYSIS This section brings together the estimates of cost and effectiveness developed for onchocerciasis control in Upper Volta and presents an illustrative comparison with similar cost-effectiveness estimates for measles immunization in Ivory Coast and Zambia. Four different measures of effectiveness are used in the comparison: years of healthy life added, productive years of healthy life added, discounted years of healthy life added, and discounted produc- tive years of healthy life added. Onchocerciasis control The essence of the procedure for estimating the cost-effectiveness of onchocerciasis control is to discount the costs and effectiveness over the life of the control project, assumed here to run for 20 years from 1975 to 1994. In order to highlight the effect of intro- ducing a discount rate, the effectiveness measures are presented in both undiscounted and discounted form. Costs are discounted in all cases. It was estimated previously that the cost of the Onchocerciasis Control Programme that should be attributed to onchocerciasis control in Upper Volta is US$2.6 million per year at 1977 prices. Assuming a 10%o discount rate, the sum of the discounted value of these costs over 20 years is US$22.1 million. Our estimates of the effectiveness of the pro- gramme are based on a projection to 1994 of the series of cases of blindness prevented given in Table 4. The numbers of cases prevented are transformed into years of healthy life added using the assumptions underlying Table 5. Thus, the total number of years of healthy life added by preventing blindness over the 20-year life of the project is 1 098 095. Since all these additional years fall within the productive age group of 15-59 years, the total number of productive years of healthy life added is also 1 098 095. Total dis- counted years of healthy life added are estimated by discounting future years of healthy life added to their value in the pre-project year 1974. The relevant formulae are as follows: Hyperendemic areas: 20 ((2-r) 23 n = I Qi (12+ri)) Mesoendemic areas: 20 t 20 L Qtn = 1 \f A where r denotes the discount rate (expressed in decimals), and Qt the number of new cases of blindness prevented in year t. In each formula, the second summation represents the number of years of healthy life added by preventing blindness in year t, discounted to year t. The first term discounts this number back to its value in the pre-project year. Assuming a discount rate of 10%, the total dis- counted number of both years of healthy life and pro- ductive years of healthy life added is 147 294. These results yield the following cost-effectiveness estimates for blindness prevention by onchocerciasis control: US$20 per year of healthy life and per productive year of healthy life added, and US$150 per discounted year of healthy life and per discounted productive year of healthy life added. Measles immunization Shepard (10) and Ponnighaus (4) have prepared cost-effectiveness estimates for mortality reduction through measles immunization in Ivory Coast and Zambia, respectively. Shepard estimated the cost per measles death averted at US$479 at 1978 prices. Assuming that the average age of death from measles is 1 year, and that the expectation of life at age 1 is 46 years, this implies a cost per healthy year of life added of US$10. However, the initial 14 years of this gain in life expectancy are non-productive. This leaves a gain in productive life expectancy of 33 years, implying a cost per productive year of healthy life added of US$15. At a discount rate of 10010, the discounted years of healthy life added per death averted are 9.88 years. However, only 2.52 discounted productive years of healthy life are added, since the gain in pro- ductive years does not begin to accrue until age 15. Thus, the cost per discounted year of healthy life added is US$49, and the cost per discounted pro- ductive year of healthy life added is US$190. Ponnighaus's estimate of the cost per measles death averted is slightly higher, equivalent to approximately US$557 in 1977 prices for a programme offering 100%o coverage to a rural population. Using the same assumptions about average age of death and life expectancy the following cost-effectiveness estimates are obtained: US$12 per year of healthy life added, US$17 per productive year of healthy life added, US$56 per discounted year of healthy life added, and US$221 per discounted productive year of healthy life added. Comparison of cost-effectiveness The different cost-effectiveness estimates for onchocerciasis control and measles immunization are brought together in Table 7. These comparisons are subject to the usual, unavoidable reservations about intercountry comparisons based on international 800 COST-EFFECTIVENESS OF BLINDNESS PREVENTION BY THE OCP Table 7. Comparison of estimated cost-effectiveness of onchocerciasis control and measles immunization Cost of measles Cost of immunization oncho- cerciasis Ivory control Coast Zambia (US$) (US$) (US$) Per year of healthy life added 20 10 12 Per productive year of healthy life added 20 1 5 1 7 Per discounted year of healthy life added 150 49 56 Per discounted productive year of healthy life added 150 190 221 exchange rates. They indicate that the relative cost- effectiveness of onchocerciasis control is very sensi- tive to the choice of effectiveness measure. Without differential weights for productive years and social time preference, onchocerciasis control appears to be a less efficient use of resources than measles immuniz- ation. The separate introduction of these weights in the measures of productive years and discounted years of healthy life added does not alter this policy inference. However, the combination of these weights in the measure of discounted productive years of healthy life added switches the cost-effectiveness ranking in favour of onchocerciasis control. The reason is that blindness prevention results in an im- mediate gain of productive years whereas the gain of productive years generated by measles immunization is deferred for 14 years, and is therefore heavily dis- counted. We consider that the introduction of weight- ing for age preference and of discounting for time preference are both consistent with conventional social values. Thus we conclude that onchocerciasis control may be compared favourably with measles immunization. It should also be remembered that our analysis focuses on blindness only and thus excludes additional health and economic benefits of oncho- cerciasis control, which may be significant. Our measure of health benefits does not include the avoidance of periods of infection without ocular involvement or with only partial visual impairment, which usually precede the onset of blindness. Similarly, our emphasis on cost-effectiveness analysis sets aside the potential benefit of increased output resulting from increases in the supply of labour and land that might be attributable to onchocerciasis control. RESUME LE RAPPORT COOT/EFFICACITA DE LA PRtVENTION DE LA CEC1Tt EN HAUTE-VOLTA SOUS L'ACTION DU PROGRAMME DE LUTTE CONTRE L'ONCHOCERCOSE L'approche retenue pour mesurer le rapport couit/ efficacit6 de la pr6vention de la cecite en Haute-Volta sous l'action du Programme de lutte contre l'onchocercose est bas&e sur une estimation du nombre d'annees productives gagn6es par la communaut6 lorsqu'on evite l'invalidit6 totale et le d6ces pr6mature lies a la cecite. Cette approche limitee a ete prefer6e a une veritable analyse cout/benefice qui aurait fait intervenir des hypotheses difficilement verifiables telles que l'influence de la baisse progressive de l'acuite visuelle sur le rendement du travail, ou que I'augmentation des surfaces cultiv6es sur les terres dites liber6es de l'onchocercose. Les donn6es de base sont constitu6es par l'ensemble des informations 6pid6miologiques recueillies depuis dix ans sur les c6cites en Haute-Volta: prevalence et incidence de la c6cit6 avant d6clenchement de la lutte contre l'oncho- cercose, effectif des populations a risque, pr6valence des c6cit6s non-onchocerquiennes, r6duction de l'esp6rance de vie des aveugles en fonction de l'Age d'acquisition de l'infirmitM, r6duction de l'incidence observ6e depuis l'arret de la transmission de la maladie. Ces donn6es montrent que l'onchocercose etait responsable d'environ 2400 cas de c6cite par an en 1975, date de debut des op6rations de lutte, et que l'incidence est devenue negligeable depuis 1981. La durke de vie moyenne des aveugles etant de 7 a 9 ans, selon les niveaux d'endemicite, et plus courte de 7 a 14 ans que celle des individus non aveugles, un calcul detaille montre que le b6n6fice de la prevention de la c6cite onchocerquienne en Haute-Volta est chaque annee d'environ 60 000 annees- homme productives depuis 1981. L'approche utilisee par les auteurs innove en ce qu'elle discute la n6cessite de ponderer de tels resultats bruts par des jugements de valeur de nature sociale: une ann&e d'infirmitM est-elle socialement et 6conomiquement equivalente a une ann&e perdue par mort pr6matur6e? Une ann&e perdue a- t'elle la meme valeur dans la tranche d'age productive (15 a 60 ans) que dans les autres tranches et specialement chez les enfants? Les ann&es productives gagn&es ont-elles la meime valeur sociale quelle que soit la date a laquelle elles sont acquises & la societe, ou y a-t-il une ponderation a effectuer en faveur des avantages immediats? En d'autres termes, est- il 6quivalent pour une communaut6 qu'une action de sant6 sauve cent vies cette ann&e ou cent vies dans dix ans? Apres discussion, les auteurs proposent de conf6rer a la cecit6 le meme poids qu'au decs premature, de conf6rer un poids nul aux ann6es gagn6es dans les tranches d'age non productives, et de conf6rer une valeur actualisee aux avantages futurs par l'application d'un taux d'actualisation 801 802 A. PROST & N. PRESCOTT de 10% par an, chiffre relativement classique en analyse 6conomique. Ils concluent que le coOt unitaire par ann&e-homme productive gagn&e par la Haute-Volta au cours des vingt annees de la duree prevue du Programme de lutte contre l'onchocercose (1975-1995) est de 20 $ E.U. en valeur brute, et de 150 $ E.U. en valeur actualisee. 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