A 99.9% Survival Rate?! How to Understand Disease Calculations and Interpretations
If I think back to when the pandemic first started, I remember being surprised when I started hearing so many people use epidemiological terminology such as mortality rate, survival rate, and more. In my home, we talk about public health ALL THE TIME. Dinner table discussions about the burden of disease, access to care, and brainstorming innovative public health solutions are common. In fact, just for fun, my oldest son and I would draw epi maps of disease distribution and calculate R0 as we learned more and more about COVID-19.
As the pandemic continued, I quickly realized that even though public health terminology was being thrown around, we were not speaking the same language as I heard so many calculations and analyses misinterpreted. Mortality rate was the calculation I heard misrepresented most often. Did you ever hear someone say that COVID has a 0.1041% death rate? Or did you ever hear the opposite of that, which was presented as COVID has a 99.8959% survival rate? I heard those so often! I remember the first time I heard a news channel interview physicians who were stating that COVID has a 99.89% survival rate, and I was in utter shock! As news with incorrect biostatistical calculations began to spread, the questions started rolling in from family, friends, students, and colleagues.
To this day, I still get several questions about understanding mortality rate (death rate) and survival rate. I teach courses such as applied epidemiology where students have to learn those calculations, but I also try to incorporate them in other public health classes so that students can identify when they may be hearing fake or misinterpreted health news. Let’s look at what mortality rate is, how it is calculated, how you should hear it interpreted, and what survival rate refers to so that you can be ready to understand this terminology when you hear about diseases.
What is mortality rate?
Mortality rate is simply a calculation used to understand the rate of death within a population. You may hear it also called death rate, and that is fine. Sometimes we look at mortality rate by disease to learn about how a specific disease is impacting us. We can look at mortality rate by age, gender, race, location, and more. It is an incredible tool that gives insight as to who is dying from what and at what rate so we can learn to prioritize and prevent (when possible).
How is mortality rate calculated, interpreted, and expressed?
Mortality rate is calculated by taking the number of deaths from a disease or cause and dividing by the total population. It is most often calculated in 1-year periods of time so that we can compare one year with the previous for change, and because we want rates to have consistent intervals. The key to mortality rate is that it should never be expressed as a percentage! It should be interpreted as a number per 100,000 people. If you ever heard COVID mortality expressed as a death rate of 0.1041% then that is completely incorrect! We do not use percentages with mortality rate because every disease would look so similar and it would remove the human aspect of disease. I will highlight the top 5 causes of death for adults in the U.S. (2021) below and show the correct mortality rate and the incorrect percentages so you can see the difference.
Top 5 Causes of Adult Deaths in the U.S. in 2021 (CORRECT MORTALITY RATE)
Heart Disease: 173.8 deaths per 100,000 people
Cancer: 146.6 deaths per 100,000 people
COVID-19: 104.1 deaths per 100,000 people
Unintentional Injuries: 64.7 deaths per 100,000 people
Stroke: 41.1 deaths per 100,000 people
Top 5 Causes of Adult Deaths in the U.S. in 2021 (INCORRECT MORTALITY RATE as a percent)
Heart Disease: 0.1738%
Cancer: 0.1466%
COVID-19: 0.1041%
Unintentional Injuries: 0.0647%
Stroke: 0.0411%
All deaths from diseases are a fraction of a percent if you divide them. The key in biostatistics, epidemiology, and public health data is that the interpretations and results HAVE TO BE TRANSLATABLE TO THE POPULATION. You have to be able to make the connection from the disease calculations to how they are impacting actual people so that programs can be designed to prevent and stop the spread of disease when possible. Our brains can’t translate a death rate of 0.1738% to the population. But we can certainly understand and have context when mortality rate is expressed as 173.8 deaths per 100,000 people. We can comprehend what 100,000 people looks like and picture that almost 174 of them (173.8) are dying from heart disease each year.
*Also, as a side note, we never use just one disease calculation to understand a disease. We would never just state the mortality rate alone and act like we had a full understanding of disease transmission. As we have all seen with COVID, diseases can mutate, mitigation efforts can slow the spread, and other factors influence the incidence and prevalence rates. We need several calculations to give us insight and understanding, and mortality rate is just one of many. I always picture diseases as a canvas painting, and each epidemiological calculation is a new color to paint more of the picture.*
Ebola is an excellent example of why we not only use mortality rate in terms of death per 100,000 instead of a percentage, but also why we use other calculations to paint a better picture of disease. Guinea suffered from an Ebola outbreak that began in 2014. Approximately 3,814 people were infected and 2,544 died from the disease. The population of the country at the time was about 11.3 million people. Here are what the correct and incorrect expressions of mortality rate would be, and I will add case fatality rate (which IS A PERCENTAGE) so you can see why we use multiple calculations to understand the impact of disease on a population.
CORRECT Ebola Mortality Rate (Guinea Outbreak 2014):
22.5 deaths per 100,000 people
INCORRECT Ebola Mortality Rate (Guinea Outbreak 2014):
0.0225%
CORRECT Case Fatality Rate (Guinea Outbreak 2014):
66.7016% case fatality rate (CFR)
If we look at those disease calculations, we can quickly see that 22.5 deaths per 100,000 people would likely be in the top 10 causes of death. If we see the incorrect interpretation, 0.0225% death rate, then that doesn’t mean much, cause alarm, or help us understand the population. Case fatality rate is what helps us understand the urgency and severity. It shows us that almost 1 in 3 people (66.7%) who got the disease died! Mortality rate is helpful to see death over time and in proportion to the population and other diseases, but case fatality rate gives us the weight of how many infected people are dying. Can you imagine if Ebola was presented to the people of Guinea as, “Oh, don’t worry, Ebola has a death rate of 0.0225%, you have nothing to be alarmed about right now.” Or if an incorrect survival rate was stated (like we saw with COVID) as, “Don’t worry, Ebola has a 99.98% survival rate!” But the disease was actually killing 1 in 3 infected people! That is why we only express mortality rate as death per 100,000 people, not a percentage, and survival rate is not calculated or expressed that way.
Please do not listen to news or sources if they are presenting mortality rate as a percentage instead of a rate per 100,000 people. We can’t lose population translation when we are learning to understand diseases. Also, please do not listen if someone tries to flip the incorrect mortality percentage and shares it as a “survival rate”. We typically use survival rate in clinical trials to see if treatment options worked (looking at the survival of patients with treatment vs without), and in explaining cancer survival (usually in 1- and 5- year increments). Survival rate is not usually used in general disease expression, nor is it calculated by just flipping the mortality rate.