Since the beginning of this COVID-19 epidemic, the terms “mortality rate” and “death rate” due to the virus have been used in almost every article, op-ed, and scientific papers. Further, they have been used interchangeably to mean both “mortality rate” and “case fatality rate”. In epidemiology, however, these terms have specific meanings and we need to use them correctly to ensure clarity in communication.
In very simplified terms:
- Mortality Rate = Death Rate for a POPULATION
- Case Fatality Rate = Death Rate for total CASES
Mortality Rate (synonymous to Death Rate): This measure is a rate calculated on a population basis. The population could be defined for the world, a country, a state, or a specific community.
For a Mortality Rate, the number of deaths is the numerator, and the number of individuals in the population is the denominator, and is typically expressed as “per 100,000 population.”
Let’s look at the world, the US, and Sweden as examples to calculate mortality rates. From data accessed on May 5, 2020 from worldometers.info, we have:
- World: 256,798 deaths/7,782,459,435 population
- US: 71,548 deaths/331,002,651 population
- Sweden: 2,854 deaths/10,099,265 population
- World Mortality Rate: 3.3 per 100,000 population
- US Mortality Rate: 21.6 per 100,000 population
- Sweden Mortality Rate: 28.3 per 100,000 population
A mortality rate calculated in this manner is a Crude Mortality Rate. Note: While it is tempting to use crude mortality rates to compare different populations with each other, because mortality rates are highly associated with older ages, the mortality rates must first be Age-Adjusted. Age-Adjusted Mortality Rates take into account the different age distributions of a population.
Another erroneous use of “Mortality Rate” is when “Case Fatality Rate” is intended. To calculate a Calculate Fatality Rate (CFR), the number of deaths are in the numerator and the number of cases are in the denominator. Note: CFR actually is not a rate but a ratio as it is a proportion and not a rate.
With COVID-19, we have seen varied CFRs which is not unexpected given variations in the populations and the disease and incomplete data especially with the actual number of cases which makes it challenging to know the “true” CFR.
Given growing evidence for asymptomatic infection, Infection Fatality Rate (also a ratio) is also an interesting measure. With number of deaths in the numerator and the number of infected persons in the denominator. There are challenges to calculating this as well, especially with the current state of antibody testing and what the presence of (or lack thereof) antibodies actually means.
These different measures of death frequency—with other data, of course—for COVID-19 help give us important information with which to make medical, public health, and policy decisions.
For further reading on age-adjustment: See Age-Adjusted Rates – Statistics Teaching Tools from the New York State Department of Health