Oklahoma Interim State Epidemiologist Jared D. Taylor explains what the mortality rate tell us about the coronavirus.
1. What is mortality rate?
This reflects what proportion of the total population dies from a given condition during a certain time interval (usually a year). This is usually expressed as "number of deaths per 1 million people."
When considering the mortality rate of COVID 19, and particularly comparing it to other diseases, it is imperative to realize that there has not been a full year of COVID-19 deaths in the US. That means that the only accurate way to compare COVID-19 to other diseases is to adjust the numbers to account for the different time intervals (i.e., look at # of deaths per day or month, or adjust COVID-19 deaths to what the current # of cases would translate to over a full year).
The formula for mortality rate is: (# of deaths due to the disease) / (population at risk for the time period in question). For most causes of disease, the population at risk is the entire population of the county, state, or nation (exceptions would be prostate cancer, ovarian cancer, etc.).
2. What is case fatality rate?
This reflects what proportion of people who are diagnosed with the disease ultimately succumb to the disease. This is generally considered to reflect the severity of the disease. In relation to COVID-19, the challenge in accurately characterizing the case fatality rate is the fact that diagnosis via testing has varied tremendously throughout the world, and over time within a given region and nation, throughout the pandemic.
When testing was extremely limited, only the very severely affected were being tested. Many of these who were severely affected died from the condition, while many who were infected but not tested or diagnosed, recovered. This gave the appearance of a high case fatality rate.
As testing has become more readily available, testing has expanded to include many people who are less severely affected (and in some cases, no obvious symptoms at all). This has led to a notable decline in "case fatality rate."
Concurrent with this increase in testing, though, has been improved case management, which would also be expected to decrease case fatality. As such, it is difficult to determine exactly what degree of improvement in the case fatality rate is attributable to various possible explanations. The formula for calculating the case fatality rate is (# of deaths due to the disease) / (# of people diagnosed with the disease).
3. What is infection fatality rate?
This reflects what percentage of people infected with the agent ultimately die due to the disease. COVID-19 is relatively unique in that many individuals infected with the causative virus (SARS CoV-2) develop very mild (and vague) or even no discernible symptoms. When this occurs with other diseases, we typically don't actively seek to diagnose those infections (sometimes we do- such as with sexually transmitted infections, but not always). That means that we don't commonly examine or discuss what percentage of people infected with an infectious agent die, regardless of development of symptoms.
With COVID-19, we have sought to diagnose infected people, regardless of presence/absence of symptoms. As we have included more people who don't have symptoms in our denominator, the infection fatality rate has declined. However, many people have sought to compare COVID-19's INFECTION fatality rate to the CASE fatality rate for other diseases. This isn't a fair or accurate comparison to make.
Calculation of infection fatality rate is: (# of deaths due to the disease) / (# of people diagnosed as infected, regardless of symptoms)
4. Why does the mortality rate for COVID-19 vary throughout the world, country and state?
The reasons for this variation include when the virus was introduced to the population; how broadly it spread before detection and control efforts began; the susceptibility of the population; uptake and efficacy of mitigation efforts; accuracy of diagnosis (in terms of those who may have died from the disease); and other considerations.
In Oklahoma, the virus was introduced a little later than in other locations, and we had the opportunity to prepare a little better than some other locations. The "Safer at Home" period was also very effective in reducing the spread of virus and greatly reduced the impact that would have otherwise been seen.
With all of that said, Oklahoma has still seen an average of over 4.5 deaths per day since the first COVID-19 death occurred in the state (in a population of just under 4 million, or roughly 1.1 death per 1 million residents, per day). If that rate held steady for an entire year, it would likely put it as the 8th most common cause of death in the state.
5. How does the COVID-19 mortality data compare to that of the flu?
Work that has been done in circumstances where extensive testing was possible for a full population (cruise ships; extensive contact tracing; etc.) would suggest that COVID-19 has an infection fatality rate of 0.5% (of course, this can vary depending upon the general health of the population; how overwhelming the viral load is in various circumstances; case management; etc.).
While this may seem somewhat low, it is 5 to 10 times higher than the typical CASE infection rate for influenza (which can vary year to year, depending upon the virulence of the pervasive flu strain for a given year). Comparing that to the INFECTION fatality rate for flu is harder, but is very likely to be 10 times higher (or more) than for influenza. As such, COVID-19 most assuredly is a severe disease that should be taken very seriously.
Finally, it is important to note that the numbers offered above are for all individuals in a population. The case fatality rate of COVID-19 is notably higher in individuals at high risk of severe disease (elderly, and those with other chronic health conditions). In populations at highest risk for severe disease and death, the case fatality rate could easily be 15% or higher.