Mortality rate vs Case fatality rate: They are often confused in media reports covering outbreaks, even by doctors who have turned into in public health spokespeople, yet they are not the same thing- and that matters.
To explain the difference and why it matters, I used Bird Flu, since it’s 1) in the news and 2) being misrepresented in major media outlets. But the differences in mortality rate vs case fatality rate would apply to any disease. I also made a video to explain the differences and used kitchen supplies as props. If you know me, you’ll know that I am a fan of presenting public health or epidemiological topics to the general public using kitchen supplies. These concepts can be difficult to grasp, especially if you don’t work in the epi. field, so I figure “showing you” the concepts, using familiar supplies in the average kitchen, will make them easier to grasp. The video is posted on my Youtube channel, but embedded here. I hope you consider subscribing to my channel! Below the video is the transcript (everything I’m saying) to help drive home the key points. (Of course, if you have any questions, feel free to email me: erin@bloomingwellness.com.)
Video Explanation:
Transcript:
One thing I hope the media improves on when it comes to scientific communication is recognizing that the stories behind the numbers matter just as much as the numbers themselves. Take bird flu as an example. You’ve probably seen the number 52% thrown around as the mortality rate of bird flu. But that’s not accurate. That 52% actually represents the case fatality rate (CFR)—a measure of deaths among people diagnosed with bird flu and reported to the World Health Organization (WHO) between January 2003 and May 2024.
Here’s how they got that number: There were a total of 889 cases reported and 463 deaths. 463 divided by 889, times 100 = 52% case fatality rate.
Now, the mortality rate is very different. It’s the number of deaths from a disease in the entire population. Think about your whole town. The mortality rate is like asking: “Out of everyone in town, how many people died from bird flu this year?”
So if bird flu only infects a small portion of the population, the overall mortality rate will be much lower than the case fatality rate.
But here’s the other thing about case fatality rates—they’re not set in stone. They can vary widely depending on: New information as it comes in; Where the cases are being reported from; The health of the affected population; Testing availability, and How a “case” is defined. (These are not all the factors that can impact the case fatality rate but key ones.)
Whenever I see a case fatality rate, my first thought is: “How was a case defined?” And my second thought might be: “Where are they being reported from?”
For example, let’s say WHO defines a bird flu case as “having a fever and a positive test.” Now, imagine an outbreak happening in a rural or underserved area, where people don’t or can’t go to the hospital. Or perhaps there’s no widespread testing because the town is poor. In that situation, the only people who get tested are the ones sick enough to end up in the hospital. Meanwhile, the milder cases—the sniffles, the runny eyes—just go unrecorded. That means the case fatality rate will be artificially high, because you’re missing all the mild cases that would have made the denominator larger.
This is why a case fatality rate in one country might be very different from another. A disease like Mpox, for example, might hit people harder in areas with high malnutrition and poor healthcare access, while in a developed country with quality healthcare, the CFR would be much lower.
Now, the opposite can happen too—you can deflate the case fatality rate. Say there’s no widespread testing for a new virus, and you define a case as “productive cough, fever, and difficulty breathing.” Well, a lot of things cause those symptoms, but you’re now labeling them as cases of the new virus. That could lead to misclassification and make the CFR artificially low.
A simple way to think about this: Imagine the actual cases are spoons. A narrow case definition only catches the most severe cases—the ones more likely to die—so you miss all the mild cases. That inflates the CFR. A broad case definition catches everything—all the spoons but also some forks and knives (cases of something else), which deflates the CFR.
This is why the story behind the numbers matters just as much as the numbers themselves. And as of now, in the U.S., all bird flu cases have been mild and mostly confined to people with an occupational risk—like those working with infected chickens or dairy cows. So far, there’s no evidence of sustained human-to-human transmission. Could that change? Sure. But as of now, this is the reality.
Thank you for reading and/or watching. Reach out with any questions. And if you want to know the ins and outs of Bird Flu from an international bird flu expert, please check out my podcast with Dr. Eskild Petersen here. You will not regret that you did and afterwards, impress all your friends with your bird flu trivia.
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