In 1993, the World Bank, WHO and the Harvard School of Public Health carried out a study to assess the global burden of disease for the year 1990. The methods and findings of the 1990 GBD study have been widely published (1-3). To prepare internally consistent estimates of incidence, prevalence, duration and mortality for almost 500 sequelae of the diseases and injuries under consideration, a mathematical model, DisMod, was developed
(4). The main purpose was to convert partial, often nonspecific, data on disease and injury occurrence into a consistent description of the basic epidemio-logical parameters.
Many conditions including neuropsychiatric disorders and injuries cause considerable ill-health but no or few direct deaths. Therefore separate measures of survival and of health status among survivors needed to be combined to provide a single, holistic measure of overall population health. To assess the burden of disease, the 1990 GBD study used a time-based metric that measures both premature mortality (years of life lost because of premature mortality or YLL) and disability (years of healthy life lost as a result of disability or YLD, weighted by the severity of the disability). The sum of these two components, disability-adjusted life years (DALYs), provides a measure of the future stream of healthy life (years expected to be lived in full health) lost as a result of the incidence of specific diseases and injuries (2). One DALY can be thought of as one lost year of healthy life and the burden of disease as a measure of the gap between current health status and an ideal situation where everyone lives into old age free from disease and disability.
The results of the 1990 GBD study confirmed that noncommunicable diseases and injuries were a significant cause of health burden in all regions of the world. Neuropsychiatric disorders and injuries in particular were major causes of lost years of healthy life as measured by DALYs, and were significantly underestimated when measured by mortality alone (2).
The 1990 GBD study represented a major advance in the quantification of the impact of diseases, injuries and risk factors on population health globally and by region. Government and nongovernmental agencies alike have used these results to argue for more strategic allocations of health resources to disease prevention and control programmes that are likely to yield the greatest gains in terms of population health. Following publication of the initial results of the GBD study, several national applications of its methods were used, which led to substantially more data in the area of descriptive epidemiology of diseases and injuries.
As a follow-up to the 1990 GBD study, WHO undertook a new global assessment of the burden of disease for the year 2000 and subsequent years in 2002. The GBD 2000 study drew on a wide range of data sources to develop internally consistent estimates of incidence, health state prevalence, severity and duration, and mortality for over 130 major causes, for 14 epidemiological subregions of the world (5).
Projections of global mortality and burden of disease
In order to address the need for updated projections of mortality and burden of disease by region and cause, updated projections of future trends for mortality and burden of disease between 2002 and 2030 have also been prepared by WHO (6). These have been based on methods similar to those used in the original GBD 1990 study, but use the latest available estimates for 2002 and the latest available projections for HIV/AIDS, income, human capital and other inputs (7). Relatively simple models were used to project future health trends under various scenarios, based largely on projections of economic and social development, and using the historically observed relationships of these with cause-specific mortality rates.
Rather than attempt to model the effects of the many separate direct determinants or risk factors for diseases from the limited data that are available, the GBD methodology considered a certain number of socioeconomic variables including: average income per capita, measured as gross domestic product (GDP) per capita; average number of years of schooling in adults, referred to as "human capital"; and time, a proxy measure for the impact of technological change on health status. This latter variable captures the effects of accumulating knowledge and technological development, allowing the implementation of more cost-effective health interventions, both preventive and curative, at constant levels of income and human capital. These socioeconomic variables show clear historical relationships with mortality rates, and may be regarded as indirect, or distal, determinants of health. In addition, a fourth variable, tobacco use, was included in the projections for cancer, cardiovascular diseases and chronic respiratory diseases, because of its overwhelming importance in determining trends for these causes.
Projections were carried out at country level, but aggregated into regional or income groups for presentation of results. Baseline estimates at country level for 2002 were derived from the GBD analyses published in The world health report 2004 (8). Mortality estimates were based on analysis of latest available national information on levels of mortality and cause distributions as at late 2003. Incidence, prevalence, duration and severity estimates for conditions were based on the GBD analyses for the relevant epidemiological subregion, together with national and sub-national level information available to WHO. These baseline estimates represent the best estimates of WHO, based on the evidence available in mid-2004, and have been computed using standard categories and methods to maximize cross-national comparability.
Limitations of the Global Burden of Disease framework
By their very nature, projections of the future are highly uncertain and need to be interpreted with caution. Three limitations are briefly discussed: uncertainties in the baseline data on levels and trends in cause-specific mortality, the "business as usual" assumptions, and the use of a relatively simple model based largely on projections of economic and social development (9).
For regions with limited death registration data, such as the Eastern Mediterranean Region, sub-Saharan Africa and parts of Asia and the Pacific, there is considerable uncertainty in estimates of deaths by cause associated with the use of partial information on levels of mortality from sources such as the Demographic and Health Surveys, and from the use of cause-specific mortality estimates for causes such as HIV/AIDS, malaria, tuberculosis and vaccine-preventable diseases. The GBD analyses have attempted to use all available sources of information, together with an explicit emphasis on internal consistency, to develop consistent and comprehensive estimates of deaths and disease burden by cause, age, sex and region.
The projections of burden are not intended as forecasts of what will happen in the future but as projections of current and past trends, based on certain explicit assumptions and on observed historical relationships between development and mortality levels and patterns. The methods used base the disease burden projections largely on broad mortality projections driven to a large extent by World Bank projections of future growth in income per capita in different regions of the world. As a result, it is important to interpret the projections with a degree of caution commensurate with their uncertainty, and to remember that they represent a view of the future explicitly resulting from the baseline data, choice of models and the assumptions made. Uncertainty in projections has been addressed not through an attempt to estimate uncertainty ranges, but through preparation of pessimistic and optimistic projections under alternative sets of input assumptions.
The results depend strongly on the assumption that future mortality trends in poor countries will have the same relationship to economic and social development as has occurred in higher income countries in the recent past. If this assumption is not correct, then the projections for low income countries will be over-optimistic in the rate of decline of communicable and noncommuni-cable diseases. The projections have also not taken explicit account of trends in major risk factors apart from tobacco smoking and, to a limited extent, overweight and obesity. If broad trends in risk factors are towards worsening of risk exposures with development, rather than the improvements observed in recent decades in many high income countries, then again the projections for low and middle income countries presented here will be too optimistic.
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