How deadly is COVID-19?

NCSE Executive Director Ann Reid, formerly a research biologist who helped sequence the 1918 flu virus, attempts to explain mortality rates in relation to COVID-19.


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What is the mortality rate for COVID-19, the disease caused by the novel coronavirus, SARS-CoV-2?

This question is in the news a lot this week — you can see a typical example in this article from the Washington Post.

The first thing for your students to understand is that mortality rate is determined by a very simple bit of math:

Number of deaths / Number of cases = Case Specific Mortality Rate (CSM).

Have your students try some simple examples:

What is the CSM if there are 10 deaths out of 100 cases? What is the CSM if there are 10 deaths out of 1000 cases?

Now. What if there are 10 deaths and — at first — you think there were 100 cases? But later you find out there were actually 1000 cases! Then, the CSM turns out to be much lower than you thought. That would mean that the virus was 10 times less deadly than you thought. Good news!

The bottom line is that the question of how deadly the SARS-CoV-2 virus is can’t really be answered right now. But what your class should realize is that this isn’t a question of the virus becoming more or less deadly — it’s a matter of how many cases are being detected. The CSM is likely to drop as more-and-more mild, asymptomatic, infections are detected.

You can predict that the number of cases will go up in the next few weeks as doctors all around the country are going to be testing more people. The number of deaths caused by the virus will go up, too. You’ll see discussions about estimates of the mortality rate, but the actual number will not be known for a long time.

Here’s what we know for sure: COVID-19 can be a very serious disease, but almost exclusively among a certain demographic. People 60+ years of age, especially if they have other health conditions like heart disease, are most at-risk for getting seriously ill. Therefore, even if we later find out that the CSM is much lower than the currently estimated 2-3%, the more we can reduce the spread of the disease, the fewer people will get sick, or even die.

While, thankfully, the majority of media headlines are saying that the virus might not be as deadly as initially thought, that does not mean we shouldn’t do everything we can to keep it from spreading. We want to keep that denominator as low as possible! 

Bonus question: Are there multiple strains of the virus?

Towards the end of the Washington Post article, Jeffery Taubenberger, a researcher at the National Institute of Health (and my colleague for many years on the 1918 flu project) is asked about new reports of different strains of the novel coronavirus — one less deadly than the other.

Note his answer:

  • It’s certainly possible there’s more than one strain because viruses that jump from animals to humans, as this one is thought to have done, often do change because they face different selection pressures in the new host(evolution, for the win!).


  • It’s way too early to say for sure (in other words, this is a plausible claim, but we need more evidence: science, for the win!).
NCSE Executive Director Ann Reid
Short Bio

Ann Reid is a former Executive Director of NCSE.