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Michaud CM, Murray CJL, Bloom BR. Burden of Disease—Implications for Future Research. JAMA. 2001;285(5):535–539. doi:10.1001/jama.285.5.535
Author Affiliations: Department of Population and International Health, Harvard School of Public Health, Boston, Mass.
One overall challenge for public health and medicine in the future is
to allocate available resources effectively to reduce major causes of disease
burden globally and to decrease health disparities between poor and affluent
populations. The major risk factors for death and disability worldwide are
malnutrition; poor water supply, sanitation, and personal and domestic hygiene;
unsafe sexual behavior; tobacco use; alcohol use; occupational hazards; hypertension;
physical inactivity; illicit drugs; and air pollution. The challenge for research
in the 21st century is to maintain and improve life expectancy and the quality
of life that was achieved for most of the world's population during the 20th
Gains in life expectancy worldwide were greater during the last century
than at any other time in recorded human history. The rate of increase in
life expectancies in the first half of the century was greatest in the United
States, Europe, Australia, and New Zealand. For example, in the United States,
life expectancy was 49 years in 1900 and 66 years by 1950. In developing countries,
life expectancy increased the most between 1950 and 1990: from 40 to 63 years.1 Tragically, life expectancy at birth has decreased
in sub-Saharan Africa as a result of the HIV/AIDS epidemic (human immunodeficiency
virus/acquired immunodeficiency syndrome) and decreased to less than 40 years
in 5 countries in 1999. The gap between highest and lowest life expectancy,
women in Japan (84.3 years) and men in Sierra Leone (33.2 years), was 51.1
years (Table 1).
In the new millennium, society faces difficult choices in the allocation
of scarce resources to medical care and medical research. The challenge is
to maintain and improve life expectancy and the quality of life that was achieved
for most of the world's population during the 20th century. Estimates of life
expectancy at birth are useful indices to assess trends and the distribution
of life expectancy in the world. This type of assessment, however, masks large
inequalities in life expectancy among different population groups within countries.
For instance, a study of patterns of mortality in the United States showed
that life expectancy at birth for men in 1990 varied by 16.5 years from 61
to 77.5 years, and for women by 13 years from 70.5 to 83.5 years.2 The gap is even larger when life expectancies are
calculated for each race. For instance, the disparity between life expectancies
of American Indian or Alaskan Native men in some counties in South Dakota
and Asian women in New Jersey was 41.3 years in 1990.2
Life expectancy at birth is a critical index of human health and well-being,3 and reflects the overall mortality in a population,
but provides no information about the health of the population prior to death.
Summary measures of population health that combine information on mortality
and nonfatal health outcomes have been developed to fill this gap and to represent
population health in a single number.
While several summary measures have been developed,4
the Global Burden of Disease (GBD) study in 1990 was a major landmark in the
development and analysis of summary measures of population health.5 The GBD study group developed a new system to assess
fatal and nonfatal health outcomes—the disability-adjusted life-year
(DALY)—making it possible to estimate the burden of major diseases,
injuries, and risk factors in 8 regions of the world, and in many counties
and county clusters in the United States.
The DALY measures the gap between the actual health of a population
and a hypothetical norm; namely, a life expectancy of 82.5 years for women
and 80 years for men. DALYs for a disease or health condition are calculated
as the sum of the years of life lost due to premature mortality in the population
and the years of life lost due to disability. DALYs incorporate a discount
rate for time preference and an age-weighting factor that take into account
the higher social value given young adults in most societies. The rationale
for inclusion of time preference and age weighting has been widely debated.6-8 Although explicit, the
assumptions may be modified.5 One important
implication for policymaking is that DALYs weigh the burden of diseases of
children less than those of adults.
The DALY measure has been used by the World Health Organization, which
publishes regular updates on the GBD as a statistical annex to the World Health
Report,9 and several countries, including the
United States, have developed national burden of disease estimates.10-12 These global, regional,
and national assessments are useful for comparing the distribution of the
burden of disease, quantifying the impact of major risk factors on health,
and making projections about future disease burdens. They also provide the
denominator for comparisons of cost-effectiveness of a wide array of preventive,
palliative, and curative health interventions, and for setting priorities
for health research.9
One overall challenge for public health and medicine in the future is
to allocate available resources effectively to reduce major causes of disease
burden worldwide and to decrease health disparities between poor and affluent
populations. This article summarizes findings from recent studies on burden
of disease and discusses major implications for public health and future research.
There were 56 million deaths worldwide in 1999, and 1.4 billion DALYs
were lost to disease and injury. The burden of disease is not equitably distributed.
For example, the burden of disease disproportionately affects people in sub-Saharan
Africa, which represents 10% of the total population but 26% of total DALYs.13
The top 10 causes of disease burden are responsible for 46% of all DALYs
(Table 2). Five of the top 10
causes of DALYs primarily affect children younger than 5 years (lower respiratory
tract infections, conditions arising during the perinatal period, diarrheal
diseases, childhood vaccine-preventable diseases, and nutritional deficiencies).
Two of the top 10 causes—malaria and HIV—predominantly affect
poor populations. These 7 causes are all part of what the World Health Organization
has referred to as "the unfinished agenda" of infectious diseases, perinatal
conditions, and nutritional disorders.
The 3 remaining causes—unipolar major depression, ischemic heart
disease, and cerebrovascular diseases—are chronic disorders. The fact
that unipolar major depression ranks fifth as a source of DALYs even though
it causes few deaths underscores how assessment of both fatal and nonfatal
health outcomes affects the ranking of disease burden. Rankings based on DALYs
differ substantially from rankings based on the number of deaths (Table 3). The importance of major depression
worldwide was one of the key findings of the GBD study.
Although road traffic collisions, falls, and self-inflicted injuries
account for 6.7% of total DALYs, prevention of injuries has not been a major
part of the public health agenda in developing countries. In sub-Saharan Africa,
HIV accounted for 20% of the burden of disease in the region; malaria, tuberculosis,
and vaccine-preventable childhood disease were responsible for another 20%.
In 1990, malnutrition accounted for nearly 6 million deaths (11.7% overall)
and 220 million DALYs (15.9% overall); poor water supply, sanitation, and
personal hygiene accounted for 2.6 million deaths and 93 million DALYs; and
tobacco use accounted for 3 million deaths and 36 million DALYs (Table 4).
Similar methods have been used to analyze the burden of disease in the
United States. In 1996, 34.5 million DALYs were lost: 18.5 million for men
and 16 million for women. The major causes of DALYs differ significantly between
the United States and the rest of the world in that 9 of the top 10 causes
include injuries and noncommunicable diseases. In the United States, ischemic
heart disease ranked first for men and women, accounting for 17% of all DALYs
lost, whereas HIV ranked fourth for men. Major depression caused the second
largest loss of DALYs in women and road traffic collisions ranked second for
men. Alcohol abuse and dependence, homicide and violence, and self-inflicted
injuries were among the top 10 causes for men (Table 3).
Projections of future burden of disease and risk factors are useful
to inform policymaking. The DALYs metric can be projected on the basis of
continuing secular trends to allow a prediction of the burden of disease at
any future time. The ranking of major causes of burden worldwide is expected
to change substantially over the next 20 years. The rank order of major disease
burdens by the year 2020 is expected to be dominated by ischemic heart disease,
unipolar major depression, and road traffic collisions (Table 5). Diseases affecting mostly children—lower respiratory
tract infections, diarrheal diseases, conditions arising during the perinatal
period, and childhood vaccine-preventable diseases (eg, measles)—are
projected to decrease significantly as global immunization reaches essentially
all countries. Human immunodeficiency virus infection was projected in 1990
to increase, but was still expected to rank only 10th by 2020. Regrettably,
these projections were too optimistic, since HIV in 1999 ranked second as
a cause of DALYs worldwide. The cause of DALYs due to war is projected to
rank among the top 10 causes.
It is possible to analyze the burden of disease for any category by
addressing (1) current methods; (2) potentially cost-effective ways to extend
existing tools to more individuals (allocative efficiency); and (3) situations
for which no cost-effective tools are available and for which research is
required to develop new tools (technical efficiency). The key challenge for
public health is to reduce major projected causes of burden by increasing
allocative and technical efficiency of health systems so as to increase overall
performance, and to define areas in which research is likely to have the greatest
yield in the future.
There is a strong case for the United States to invest in health research
to reduce major causes of burden of disease that are not treatable or preventable
with the current mix of interventions and health delivery systems, both nationally
and globally. Gains in education and income have contributed to approximately
half of the health gains during the past 50 years, and the other half is the
result of the generation of new knowledge and the development of new tools
(diagnostics, drugs, and vaccines).9
First, there is a complex causal web involving socioeconomic determinants
such as income, education, employment, social capital or income inequality,
and proximal behavioral and environmental factors such as tobacco use, physical
activity, diet, and health care; and physiological factors such as cholesterol
levels, blood pressure, and genes that influence mortality or disability outcomes.14 Research is needed to understand the contribution
of proximal and distal risk factors to reduce the avoidable burden of disease
and inequalities in health, as evidenced by the gaps in life expectancy within
the United States.
Second, the perception varies greatly from reality despite the enormous
barrage of health information in the media, in advertisements, and on the
Internet. For example, 1 survey found that public perceptions of major health
risks for children were drug and child abuse, which contribute a minuscule
fraction of the burden of childhood disease compared with poverty or lack
of access to health care.15 Interdisciplinary
research is needed to further the understanding of how socioeconomic factors
contribute to morbidity and mortality and how health professionals can make
significant inroads to changing unhealthy behaviors.
Third, research is also needed to improve the overall performance of
the health system in the United States. The recent World Health Report evaluated
health care systems with respect to attainment of good health, responsiveness
to the expectations of the population, and fairness of financial contribution.
While attainment of high level of care was unparalleled, the United States
ranked 32nd on the basis of equity of distribution, and the US health care
system overall was ranked 37th worldwide.13
Despite evidence that investments in health research have been among
the most cost-effective investments over the past decades, the great imbalance
between investments in health research and the GBD first highlighted by the
Commission on Health Research for Development in 1990 persists.16
Global investments in health research in 1992 were estimated to be US $55.8
billion. Even though 85% of the global burden of disability and premature
mortality occurs in the developing world, less than 4% of global research
was devoted to communicable, maternal, perinatal, and nutritional disorders
that dominate the burden of disease in developing countries. Funding for health
research expressed as expenditures per DALYs in 1990 and 2020 illustrates
low levels of funding for malaria, tuberculosis, acute respiratory tract infections,
and diarrheal diseases, all of which disproportionately affect developing
countries (Table 6). An increase
in investments by the United States and other developed nations on major causes
of burden of diseases that disproportionately affect the poor worldwide—HIV,
tuberculosis, and malaria—is justifiable both on humanitarian grounds
and also in the enlightened self-interest of people in these developed countries.
The process of globalization that dominates the beginning of the 21st century
has created a context of global health interdependence in which many health
threats transcend national boundaries, such as the spread of reemerging and
new infectious diseases, increased microbial resistance to currently available
drugs, and bioterrorism,17 and the challenges
of financing the increasing demands of the public for better health.
Past investments in health research have been very effective. The generation
and application of new knowledge accounts for approximately half of all health
gains worldwide over the last 50 years.18 The
25-year increase in life expectancy in the United States and the enormous
gains in the quality of life over the past century confirm the value to human
health of new knowledge deriving from biomedical and public health research.
With the unfolding genomic and technological revolution, continuing investments
in research offer unprecedented opportunities to understand disease processes,
prevent intrinsic and environmental risks to health, and develop new treatments
to improve the quality of life in the United States and worldwide.