Health Analysis
What is disease burden?
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Disease burden describes how diseases, injuries and risk factors contribute to premature death and reduced health in a population.
Measures of disease burden
The Global Burden of Disease Study (GBD) is a leading international initiative to scientifically measure how more than 350 diseases, injuries, and risk factors affect health. It does this for different ages, sexes, geographical areas and over time [1-3]. The purpose of the GBD project is to provide regular, evidence-based overviews of disease burden, so that health systems can be improved and inequalities reduced.
The GBD project defines disease burden in terms of health loss, and not by whether diseases affect income, use of health services or productivity.
The GBD uses six main measures of disease burden:
- Deaths
- Years of Life Lost (YLL)
- Years Lived with Disability (YLD)
- Disability Adjusted Life Year (DALY) – the sum of YLL and YLD
- Life expectancy
- Healthy Life Expectancy (HALE)
Years of life lost (YLL) is a measure of premature death. It reflects both how many people die each year and how old they are when they die. As a result, deaths at younger ages contribute to many YLLs because more years of expected life are lost, while deaths at older ages contribute fewer YLLs because fewer expected years of life remain. YLL is calculated by multiplying each death by the expected remaining life years for a person of that age, based on a global reference for life expectancy constructed from the lowest mortality rates observed globally. The same reference is used for both men and women and for all countries.
Years lived with disability (YLD) measures health loss from living with disease or injury. It is calculated by multiplying the number of new cases of a disease each year by the severity of the disease. Severity is determined by a disability weight (DW), a number between 0 (“completely healthy”) and 1 (“a year of life lost”). This means that living with a disease or injury for one year counts as losing a fraction of a year of life, depending on how severe the condition is (for a more detailed description, see fact box). A disease results in more YLDs if it is common, long-lasting, severe, or a combination of these.
Disability-Adjusted Life Years (DALY) add together YLL and YLD. One DALY represents one year of healthy life lost due to either premature death or living with disability. DALYs provide a single, overall measure of health loss in the population.
The GBD project also calculates life expectancy and healthy life expectancy (HALE), which adjusts life expectancy by accounting for time spent in poor health. A more detailed description of the disease burden measures is given in the table below.
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Table 1: Key disease burden measures in the GBD project |
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Measure (abbreviation) |
Description |
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Years of Life Lost (YLL) |
Measure of mortality. Calculated by counting the number of deaths in a given year and summing the expected remaining years of life lost. The expected remaining life is taken from a global reference life table based on the lowest mortality rates observed globally, the same for men and women. |
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Years Lived with Disability (YLD) |
Measure of health loss from living with disease or injury. Calculated by multiplying the number of new cases in a given year (incidence) by the severity of the condition, represented by a disability weight. |
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Disability Adjusted Life Years (DALY) |
A combined measure of disease burden. The sum of years of life lost (YLL) and years lived with disability (YLD) gives the number of healthy life years lost (DALY). |
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Life Expectancy (LE) |
A measure of how long a person can expect to live. It is a hypothetical measure based on the assumption that the person lives through life with the age-specific mortality rates observed in the calendar year of calculation. |
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Healthy Life Expectancy (HALE)
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A measure of how many years a population can expect to live in good health. Calculated similarly to life expectancy, but with a deduction for health loss in each age group. |
How is disease burden calculated?
In Norway, we have a good overview of what people die from as all deaths including cause of death are reported to the Norwegian Cause of Death Registry. These data form the basis for calculating years of life lost (YLL) for Norway in the GBD project.
Calculating years lived with disability (YLD) is more challenging, because diseases and injuries differ in how often they occur, how long they last, and how severe they are. For many conditions that contribute to health loss, the available data are also limited.
To calculate health loss from a disease or injury, the GBD study needs reliable information on:
- Incidence - the number of new cases with the disease during a specific period
- Prevalence - number of cases at a given point in time who have the condition.
- Severity – how much of the condition reduces health.
In Norway, data on incidence come from several registries and health surveys, as well as published reports and studies.
The GBD study starts by using global data and global statistical models to estimate disease burden. These calculations are then carried out at the regional level and further adjusted to national and subnational levels using available country-specific data. In this process, explanatory variables and predefined assumptions (known as priors) are used to ensure consistent and robust estimates, even where data availability is limited.
This stepwise approach - from global to national level - allows results to be comparable across countries and regions. When Norwegian data are missing, estimates are partly informed by data from other Nordic countries and Western Europe. Severity is assessed using health loss weights that are the same for all countries (see fact box).
Fact box: Calculation of disability weights (DW)
For each disease and injury, the GBD project defines one or more health conditions (called sequelae) that represent the non-fatal consequences of the disease or injury.
Each health condition is given a disability weight (DW) that indicates how much the condition reduces health. For example, epilepsy in the GBD project is divided into three sequelae, each with its own disability weight: (1) epilepsy controlled with medication and without seizures, (2) epilepsy with seizures less than once a month, and (3) epilepsy with seizures once a month or more. To calculate the total health loss from epilepsy, the incidence in each category is multiplied by its disability weight, and the results are summed up.
The disability weights were developed by first creating simple, easy-to-understand descriptions of all health conditions. People from the general population in nine countries across continents, as well as respondents in an online survey, were shown pairs of descriptions, each describing a person with two different health conditions, and asked to indicate which of the two people they thought was in better health. They were also asked to compare the benefits of preventing illness versus saving lives.
Based on these responses, researchers estimated the severity of each health condition and assigned it a from 0 (completely healthy) to 1 (a year of life lost). This allows time lived with disability or injury to be measured in the same unit as years of life lost due to premature death.
Most diseases in the GBD project fall between 0.1 and 0.5. Examples of low-severity conditions include mild anemia (0.004) or hearing loss (0.01). The highest weight was assigned to an acute episode of schizophrenia (0.778) [1]. This means that 1.3 years lived with severe schizophrenia is equivalent to one year of life lost.
Examples of calculating the burden of disease
Below are some examples for calculating disease burden in Norway in 2023. Note that the calculations are simplified to show the principles behind the calculations and that the estimates presented will differ from those produced by GBD for Norway. Official GBD results for Norway can be explored using the online visualization tools at www.healthdata.org.
Example 1 (YLL):
777 women died of breast cancer in Norway in 2023. If we assume that the average age at death was 67 years, the number of years of life lost is estimated to be 18 years per person. This equates to 777 x 18 = 13,986 YLL.
Example 2 (YLD+YLL=DALY):
24,000 people were living with epilepsy in Norway in 2021. The disability weight is 0.22 (if we assume the same health loss weight for everyone regardless of severity). This equates to 24,000 x 0.22 = 5,280 YLD.
There were 91 deaths due to epilepsy in Norway that year. If we estimate the number of years of life lost to 31 years per person, this gives 91 x 31 = 2,821 YLL.
Adding YLD and YLLgives the number of DALYs attributable to epilepsy in 2021 as follows: 5,280 YLD + 2,821 YLL = 8,101 DALYs.