Here's why the difference matters...
Mortality Rate = Deaths / Population at Risk Often the denominator for mortality rate is the entire population at the beginning of some time period (usually a year), but sometimes we mean a specific sub-population (for example, Infant Mortality Rate = Infant Deaths / Live Births).
Case Fatality Rate = Deaths / Population with the Condition The case fatality rate is a special subset of mortality rates in that the rate is calculated only based on those with the condition.
Because not everyone gets every condition, Case Fatality Rates are higher--sometimes dramatically so--compared with overall Mortality Rates.
Example:
The Case Fatality Rate for measles is approximately 15%. This means that of those people who get measles, approximately 15 percent die. But we have widespread vaccination, so most people do not get measles. From 2016-2019 there were 0 measles deaths among the United States population of nearly 330 million. Thus, while the Case Fatality Rate remains high, the observed measles Mortality Rate for 2016-2019 in the United States was 0.
In other words: the risk of death is still high for those who contract measles, but the number of cases has been low due to preventative measures (like vaccines).
For COVID-19:
(Based on March 1, 2020 data from WHO)
Mortality Rate = unknown
(was 3,000 deaths / 7,000,000,000 population as of March 1, 2020 but is still rising
Note: The year-end mortality rate will be much higher because the disease is still spreading, but the current number is well below 0.001%)
Case Fatality Rate = 3,000 / 87,137 = 3.4%
Note: This estimate may be too high because not every country has implemented widespread testing, so the number-of-cases denominator may be somewhat low. Best estimates to date of COVID-19 Case Fatality Rate are between 2%-3.4%.
So when you see someone refer to 2-3.4% for the COVID-19 "mortality rate," that does not mean the virus will exterminate 3 percent of the global population. It does mean, however, that we should take the risk seriously.
We can slow the spread through social distancing, good hygiene, and (eventual) vaccine development.
