As we move through the COVID-19 pandemic, you may have encountered two terms: morbidity and mortality. These related terms are commonly used in the field of epidemiology.
Morbidity and mortality describe the frequency and severity of specific illnesses or conditions.
There’s often confusion between morbidity and mortality, so this article will help explain the difference between them, along with several examples of each term.
Morbidity is the state of having a specific illness or condition. While morbidity can refer to an acute condition, such as a respiratory infection, it often refers to a condition that’s chronic (long-lasting). Some examples of common morbidities include:
You’ll often see morbidity data presented in two ways: incidence and prevalence. Let’s dive a little deeper into each of these.
Incidence
Incidence refers to the occurrence of new cases of an illness or condition within a population over a defined period of time. It can be expressed as a proportion or a rate.
Proportion
An incidence proportion can be used to estimate the risk for developing a specific condition during a given time period. It’s calculated by dividing the number of new cases during a specific period by the population at the start of the period.
For example, let’s say that 10 people became ill with food poisoning after eating undercooked chicken at a backyard barbecue. If 40 people attending the barbecue ate the chicken, the risk of food poisoning would be 25 percent.
Rate
The incident rate is the number of new cases of a disease within an at-risk population. This helps determine how quickly a disease is spreading. It’s often expressed in units of population, such as “per 100,000 people.”
For example, say you’re studying a population of 800,000 people at risk for developing hepatitis C. After 1 year, you find that 500 of those people have tested positive for the disease.
To calculate the incident rate, you’ll divide the 500 cases by the population of 800,000. You can then say that the incident rate of hepatitis C in this population is 0.000625, or 62.5 cases per 100,000 people per year.
Prevalence
Prevalence is the proportion of a population that has a condition or illness. Unlike incidence, it includes both new and existing cases. It can either be calculated at a specific point in time or over a specified period of time.
Prevalence is often expressed as a percentage. Population units, such as “per 100,000 people,” can also be used.
You may have also come across a term that’s related to morbidity. It’s called comorbidity. It means that you have more than one illness or condition (morbidity) at the same time.
Depending on the condition, some comorbidities may be more common than others. For example, according to the Centers for Disease Control and Prevention (CDC), arthritis is often associated with other comorbid conditions like obesity and heart disease.
Knowing whether you have comorbidities can be very important in a healthcare setting. That’s because they can make a difference in the diagnosis, treatment, and outlook of an illness.
COVID-19 is a good current example of this. If you have certain health conditions (comorbidities) and you also develop COVID-19, the risk of a serious illness increases. Some examples of these comorbidities include:
Knowing if someone who’s contracted COVID-19 also has one these conditions can make it easier for healthcare providers to develop an appropriate treatment plan in an effort to prevent severe illness.
Mortality refers to the number of deaths that have occurred due to a specific illness or condition.
Mortality is often expressed in the form of mortality rate. This is the number of deaths due to an illness divided by the total population at that time.
As with morbidity, mortality rate is often expressed in population units, typically as “per 100,000 people.” Let’s look at a simple example.
In 1 year, 50 heart attack deaths occurred within a population of 40,000. To determine mortality rate, you’d divide 50 by 40,000 and then multiply by 100,000. In this population, the mortality rate due to heart attack would be 125 per 100,000 people for that year.
According to the CDC, the top 10 causes of death in the United States for the year 2018 were:
- heart disease
- cancer
- unintentional injury
- chronic lower respiratory diseases
- stroke
- Alzheimer’s disease
- diabetes
- influenza and pneumonia
- kidney disease
- suicide
It’s worth noting that for the year 2020, COVID-19 will be a significant cause of mortality. A CDC report from late October predicts that the total mortality for COVID-19 in the United States will reach 250,000 to 266,000 individuals by mid-November.
If 2020 mortality rates from other causes remain similar to the numbers from 2018, COVID-19 will be the third-highest cause of death in 2020. This puts it just behind heart disease and cancer.
Generally speaking, the number of deaths remains relatively consistent in many populations from year to year. However, deaths can increase when events such as disease outbreaks, natural disasters, or wars occur.
Simply put, excess mortality is a comparison of the number of expected deaths versus the number of deaths that actually occurred.
COVID-19 is currently a cause of excess mortality throughout the world. A recent study found that deaths in the United States increased by 20 percent in the period spanning March through July of 2020. COVID-19 was the cause of 67 percent of these deaths.
The researchers theorize that the remaining excess deaths could be due to unrecognized COVID-19 or to other disruptions caused by the pandemic, such as interrupted access to regular healthcare.
Morbidity and mortality are two terms that are commonly used in epidemiology. While they’re related, they refer to different things. Morbidity and mortality are often expressed as a proportion or rate.
Morbidity is when you have a specific illness or condition. Some examples of common morbidities are heart disease, diabetes, and obesity. You can have more than one morbidity at a time. When this happens, it’s called comorbidity.
Mortality is the number of deaths due to a specific illness or condition. Common causes of mortality in the United States are heart disease, cancer, and unintentional injuries. For the year 2020, COVID-19 will also be a significant cause of mortality.
(go to Outline)
Ideally, morbidity rates should be calculated using a population denominator, just like mortality rates are. This then corrects for any changes in the population. As a result, an increase in the size of the population which produces an increase in the overall number of cases of disease does not appear to be a change in the risk of disease.
Burundian refugee camps in western Tanzania, 1997
(Source: International Rescue Committee)
This graph shows the number of cases of malaria reported from health facilities per 1000 population per week during the first 7 months of 1997. It is readily apparent that the incidence of malaria increased during this time period and that the rate is about the same in children less than 5 years of age and older children and adults. Combining these data with those above from the same population, one can conclude that the case-fatality ratio for malaria is much higher in young children than older children and adults.
However, it is sometimes impossible to get even a rough estimate of the population size. In such cases, plotting the absolute number of cases can give you some idea of the trend if the size of population from which the cases are counted is not changing rapidly.
Goma, Zaire 1994
This graph shows only the number of cases of diarrhoea reported from health facilities in the three camps around Goma, Zaire. The population denominator was unknown. However, there was little emigration or immigration in these camps at this time. The major cause of change in the population size was the substantial mortality during this cholera and dysentery outbreak.
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Study Notes
1. Role of Epidemiology
– Epidemiology is the study of sickness, disease and death in a population
– It provides us with information on the distribution patterns of illness, disease and injury and the likely determinants of these within specific groups or populations
– This epidemiological information can then be used by governments and health organisations to analyse the effectiveness of their health services and facilities
– Some examples of epidemiological sources used to study the health of population groups are:
• SPANS (Schools Physical Activity and Nutritional Survey)
• AIHW (Australian Institute of Health and Welfare)
• ABS (Australian Bureau of Statistics)
2. Measures of epidemiology (mortality, infant mortality, morbidity, life expectancy)
Mortality
– Number of deaths in a population group or from a particular disease or over a certain time.
E.g. According to the ABS, in 2010 there were 11,704 males and 10,004 females who died of heart disease. Death rates may also be measured as a standardised rate of deaths per 100,000 of the population. (Australian Bureau Statistics, 2010)
Infant Mortality:
– Number of infant deaths in the first year of life per 1000 live births
– Is the most important indicator of health status of a population
– According to the ABS, infant mortality rates has been declining steadily since 1941
Morbidity:
– The incidence or rate of illness, disease or injury in a population group
– Illness, disease and injury decreasing quality of life, either temporarily or permanently but not result in death
– Indicators of morbidity are; records from hospitals, Doctors/Medicare and official health surveys and reports.
Life Expectancy:
– The average length of time (measured in years) that a person can expect to live from birth in a specific year.
E.g. Australians born in 2007 were expected to live 80.2 years. This is predicted from the current death rate.
– Common indicator of health status of a population
– Luckily Australia has high life expectancy due to lower infant mortality rates, declining death rates for CVD, declining rates for cancer and declining rates from traffic accidents
3. What can epidemiology can tell us:
– The prevalence of a disease(the number of cases of an illness within a specific population at a given point in time)
– The incidence of a disease (the number of new cases in a specific area over a certain period of time)
– Determine if a health problem exists and justify further investigation
– Help identify major causes of illness, disease or death to assist in addressing any existing or emerging health issues.
– Evaluate health promoting behaviour that assists in disease control or prevention.
– Determine priority issues or areas for allocation of government spending.
– Identify areas of health improvement. This would affirm successful health policies or strategies by Governments of Health-related Organisations
– Evaluate the effectiveness of prevention or treatment program
4. Who uses epidemiology measures?:
– Policy designers in all levels of government
– Manufacturers of health products (e.g. drug companies)
– Providers of health services (e.g. Doctors, Hospitals and Medicare.
5. Do they measure everything about health status?
– No. Epidemiology does not:
• tell us the severity of illness or in some cases the severity of disease
• tell us the real quality of life or the impact illness and disease has on the quality of life
• tell us why we get sick
• account for health determinants (social, economic, environmental and cultural)
• tell us why health inequities exist
• tell us how governments and health-related organisations should spend their money to best tackle health issues