Editor I read with interest the critical examination of summary measures of population health by Murray and colleagues (1). The summary measure, disability-adjusted life year (DALY), that was developed in the Global Burden of Disease Study (2) has made a central contribution to the comparative assessment of disease burden. It is aggregated from disease-specific mortality and morbidity data including an appraisal of the severity of the functional consequences of the disease. The measure makes possible comparisons between health losses due to mortality and morbidity and health losses attributable to different diseases: the addition of disability results in a more realistic measure of disease burden than that obtained from mortality alone. DALYs may be used to evaluate health policies, to compare intervention alternatives, and to assess risk factors. A recent study of the relation between funding by the National Institutes of Health and the burden of disease showed that, except for AIDS and a few other diseases, the size of the burden in the USA was strongly predictive of the amount spent on research and development when disease burden was measured using DALYs (3).
Nevertheless, the limitations of DALYs are also recognized. DALYs do not cover multiple causes and long latency periods, nor do they capture discomfort, pain, suffering, stigma, or the social and economic consequences involved in many conditions, such as the burdens that maternal deaths cause in households and communities. Murray and colleagues acknowledged that certain issues are not reflected, including average levels of population health, reductions in health inequalities, responsiveness of the health system to legitimate expectations of the public regarding the non-health dimensions of its interaction with the system, and the fairness of health system financing (1).
Application of DALYs to burden analysis for infectious diseases may be even more challenging. Traditional mortality or life expectancy measures do not reflect the burden of most non-fatal chronic infectious diseases at all, even though the impact of these diseases is obvious. The DALY measure is a significant step in the right direction as it takes into account non-fatal disease burden, but it fails to address certain unique aspects of infectious diseases so may not necessarily reflect the true picture. First, there are large proportions of asymptomatic infections that may be inaccurately attributed to non-infectious chronic diseases in mortality or even in morbidity data. For example, according to available information, as many as 7080% of individuals infected with hepatitis C virus are asymptomatic; up to 85% of those asymptomatic infections may become chronic hepatitis C, approximately 20% of which will become cirrhotic, including 5% or so who will develop hepatocellular carcinoma and eventually die of the liver damage (4, 5). However, most of those deaths are not classified as caused by hepatitis C in mortality statistics (Health Canada, unpublished data). Secondly, many infectious diseases have multiple chronic sequelae such as cancer, liver diseases and infertility, which have not been taken into consideration in burden analysis. Furthermore, the transmissibility of this group of diseases is probably its most important characteristic, but the burden that could be induced from such transmissibility has not been appropriately included in burden analyses such as the DALY measure. For example, each blood donor infected with a bloodborne pathogen may be able to spread the infection to several recipients through blood transfusion or to a larger number through blood products; among injecting drug users, one infected individual could spread a bloodborne infection to a whole network of users in a relatively short period of time; and contamination of a water or food supply by an enteric pathogen may cause infections in hundreds of individuals. Without inclusion of this aspect of infectious diseases, any analysis would result in significant underestimates of both the burden and its reduction through intervention such as vaccination, one of the most effective means.
Failure to recognize the above unique aspects of infectious diseases may in part explain the puzzling fact that the public, health care professionals and governments all express concern about infectious diseases, yet these diseases are always ranked low or at the bottom of most disease burden analyses. Measures for burden analysis should take into account the unique aspects of infectious diseases so that the derived burden estimates correctly reflect the impact of this group of diseases and can be used to evaluate the effectiveness of intervention strategies. 
Shimian Zou
Centre for Infectious Disease Prevention and Control
Health Canada, Postal Locator 0601E2
Bldg #6, Tunneys Pasture
Ottawa, Ontario, Canada K1A 0L2
tel: 1-613-946-8819; fax: 1-613-952-6668
email: shimian_zou@hc-sc.gc.ca
1. Murray CJL, Salomon JA, Mathers C. A critical examination of summary measures of population health. Bulletin of the World Health Organization, 2000, 78: 981994.
2. Murray CJL, Lopez AD, eds. The global burden of disease: a comprehensive assessment of mortality and disability from diseases, injuries and risk factors in 1990 and projected to 2020. Cambridge, MA, Harvard School of Public Health on behalf of the World Health Organization and the World Bank, 1996 (Global Burden of Disease and Injury Series, Vol. 1).
3. Gross CP, Anderson GF, Powe NR. The relation between funding by the National Institutes of Health and the burden of disease. New England Journal of Medicine, 1999, 340: 18811887.
4. Zou S, Tepper M, Giulivi A. Current status of hepatitis C in Canada. Canadian Journal of Public Health, 2000, 91 (suppl 1): S10S15.
5. Zou S, Tepper M, El Saadany S. Prediction of hepatitis C burden in Canada. Canadian Journal of Gastroenterology, 2000, 14: 575580.
Editor The above letter by Shimian Zou highlights one of the fundamental issues in the construction of health gaps such as disability-adjusted life years (DALYs), namely the problem of causal attribution. In arguing that DALYs underestimate the burden due to infectious diseases, however, Zou fails to appreciate the distinct merits of the two major approaches to causal attribution categorical attribution and counterfactual analysis which we have discussed elsewhere (1). In brief, categorical attribution assigns every event such as a death to a single cause according to a defined set of rules; this approach has the advantages of being simple and comprehensible, and provides the intuitively appealing property of additive decomposition (i.e., the total burden equals the sum of the burdens attributable to each cause). The other major tradition, counterfactual analysis, determines the contribution of a particular cause to the overall burden by comparing the current level of burden to the hypothetical level that would prevail if that cause were reduced or eliminated. While the counterfactual approach provides a conceptually clear solution to the problem of multi-causality, it is considerably more complicated to compute and less easily understood. In the example that Zou cites, relating to hepatitis C and chronic liver disease, the health outcomes are not inaccurately attributed but simply attributed categorically, according to the conventions of the International Classification of Diseases. The Global Burden of Disease Study (2) also used a simple form of counterfactual analysis (population attributable risk) to calculate the total burden attributable to various diseases and certain risk factors (such as unsafe sex) that cause other diseases after long latency periods.
Zou notes that DALYs do not capture the social and economic consequences involved in many conditions. As discussed in our paper (1), DALYs are a health gap measure that quantifies loss of health for a population against a normative standard, and are not intended to be a measure of total well-being. However, DALYs do capture discomfort, pain, suffering and stigma, as these aspects of health states are taken into consideration in measuring disability weights.
It is important to emphasize that measuring the burden of disease and assessing the potential benefits of interventions are distinct, albeit related, goals. The issue of averting future transmission is more relevant to intervention analyses than to describing a populations health during a defined period. An intervention analysis requires a dynamic application of burden assessment in which changes in an entire future stream of burden are computed in order to capture the anticipated benefits of an intervention (3). Clearly, however, even assessment of burden in the current period reflects the transmissibility of infectious diseases.
Finally, it is worth mentioning that, contrary to Zous assertion that infectious diseases are always ranked low or at the bottom of most disease burden analyses, a glance at the leading causes of DALYs globally in 1999 (4) shows that infectious diseases occupy four of the 10 highest ranks, including acute lower respiratory infections (1st), HIV/AIDS (2nd), diarrhoeal diseases (4th), and malaria (8th). Measles and tuberculosis are also in the top ten ranks for developing countries.
Joshua A. Salomon,
Health Policy Analyst
email: salomonj@who.ch
Colin D. Mathers,
Scientist
Christopher J.L. Murray,
Director
Global Programme on Evidence for Health Policy
World Health Organization
1211 Geneva 27, Switzerland
1. Murray CJL, Salomon JA, Mathers CD. A critical examination of summary measures of population health. Bulletin of the World Health Organization, 2000, 78: 981994.
2. Murray CJL, Lopez AD, eds. The global burden of disease: a comprehensive assessment of mortality and disability from diseases, injuries and risk factors in 1990 and projected to 2020. Cambridge, MA, Harvard School of Public Health on behalf of the World Health Organization and the World Bank, 1996 (Global Burden of Disease and Injury Series, Vol. 1).
3. Murray CJL, Lopez A. On the comparable quantification of health risks: lessons from the Global Burden of Disease Study. Epidemiology, 1999, 10: 594605.
4. The world health report 2000 Health systems: improving performance. Geneva, World Health Organization, 2000.