Monday, July 20, 2020

About That 99.957% Survivability Rate

"COVID-19 is not that big of a deal", you've likely seen screeching across your Facebook feed. Reassurance is offered in a phrase similar to "The Coronavirus survivability rate is 99.957%". The number on the right of the decimal often changes a little based upon which exact figures someone is using for their calculations. This "survivability rate" is calculated by taking the number of COVID-19 fatalities reported in the US (143,000 as of this writing) and dividing it by the total US population size (331,002,561). This gives us 0.0004320. To account for the percentage, we have to move the decimal two positions to the right which gives us .043% (rounded). Subtract that from 100% and you get a survivability rate of 99.957%. So, this number seems to say that your chance of living through the COVID-19 pandemic is 99.957%. You gotta like those odds, right? Unfortunately, that figure is deceptive for a number of reasons.

There are a myriad of different ways to calculate fatality rates of any given disease. The 99.957% figure is an inverse of what is called the crude mortality rate. The crude mortality rate takes the number of deaths due to a disease and divides it by the total population. Indeed, you have likely seen posts on Facebook saying "The mortality rate of COVID-19 is only .043%!". What's wrong with that number? Well, it has been about 25 years since I have taken a college-level statistics class, but it seems to me that there's some selection bias involved. To illustrate what I am getting at, let's do some comparisons of various mortality rates from 2019 using 328,200,000 for the 2019 population:

Disease Number Of Deaths Crude Mortality Rate Survivability Rate
Heart Disease 647,000 .20% 99.800%
All Cancers 606,880 .18% 99.820%
All Accidents 169,936 .052% 99.948%
Chronic lower respiratory diseases 160,201 .049% 99.951%
Stroke 146,383 .045% 99.955%
Lung Cancer 142,670 .043% 99.957%
Influenza 55,672 .017% 99.983%
Car Accidents 38,800 .012% 99.988%

So, what does this tell us? This tells us that the three leading causes of death last year each have a survivability rate of 99% (truncated). This tells us that, while we can all agree that every person has a 100% chance of eventually dying of some cause during some year, the chance of dying of any one specific cause in any one specific year is minuscule. Using the crude mortality rate, your chance of dying at all in 2019 was .9% giving you a survivability rate of 99.1% for the year (assuming 3,000,000 total deaths in the US).

Some people rail against mask mandates, social distancing and banning large gatherings for a disease with a survivability rate of 99.957%. To that, I say, heart disease has a survivability rate of 99.8% so eat all the fatty foods you want! Why are we banning smoking in bars and putting labels on cigarettes when Lung Cancer has a survivability rate of 99.957%? Why do we require seat belts when car accidents have a whopping 99.988% survivability rate? Because calculating an individual's risk of dying of a disease involves a number of factors and is much more complex than just dividing two numbers. But, even if we accept the crude mortality rate method, those tiny percentages still equal a lot of people, and, without those mandates and guidelines, the number of deaths would surely go up.

Maybe you don't see any deception or selection bias in those numbers. At the very least, the crude mortality rate doesn't indicate the likelihood of someone dying who has contracted the disease. For that number, we need to calculate the case fatality rate. The case fatality rate is the number of deaths from a certain disease divided by the number of people diagnosed with the disease. To find the case fatality rate of COVID-19 in the United States, we take number of reported deaths (143,000) divided by the number of US Coronavirus cases (3,830,000). This gives us .03733. If we move the decimal to account for the percentage, then we get a Coronavirus case fatality rate of 3.7% in the US. For comparison, the case fatality rate for influenza was less than 1% for 2019. And, keep in mind, we're only about 8 months into 2020, so the number of Coronavirus deaths will certainly increase.

As it stands, Coronavirus is on track to be the third leading cause of death this year. Just as we want to keep Heart Disease and Cancer deaths to a minimum, so too must we enact measures to keep the Coronavirus deaths to a minimum. All of the peer-reviewed science indicates that masks and social distancing offer at least some measure of protection. So, for now, keep your public interactions to a minimum. And, if you do go out in public where there's a crowd, then put on your mask. This isn't about freedom, this isn't about an unnecessary panic, this is about compassion. This is about keeping that .043%. crude mortality rate as low as possible.

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