Saturday, April 04, 2020

It's just the flu

When contrarians compare COVID-19 to influenza, they invariably mean to minimize it's significance. From what we know now this seems absurd, but of course it's not so simple.  The 1918 pandemic, after all, was just the flu -- and we don't think that's the worst influenza can be.

So how does COVID-19 compare to the spectrum of influenza? Wikipedia has an article on the CDC pandemic severity index that ranks various influenza pandemics. The 1918 pandemic was Category 5 - a case fatality rate (CFR) of 2.0% or higher. The worst influenza in my life was the Hong Kong flu with a CFR below 0.5%. It is said to have killed a million people worldwide (out of 3 billion).

The COVID-10 CFR seems to fit that range. We think its CFR is somewhere between 0.7% (based on presumed cases) and 1.5% (based on excess mortality). So by CFR it is arguably "just the flu".

What about if we look at the other half of the equation - the Basic reproduction number (R0 how contagious a disease is)? Wikipedia is again helpful; influenza ranges from 0.9 to 2.8, the early estimates for COVID-19 the 1.4 to 3.9. So COVID-19 fits the influenza model there as well, as long as we include monster events that cause historic devastation.

We can also look at who dies, and the disability of those who survive. Some influenza takes the young, some take the middle-aged, most take the old. COVID-19 seems to go for the middle-aged and old, so again flu like. As to disability, I haven't seen any reports on post-influenza disability.  I wonder if persistent lung damage will be one way that COVID-19 is not flu like. We don't know yet.

So, yeah, COVID-19 mostly fits within the spectrum of influenza, as long as we include pandemics that hit every 100 years or so. It's "just the flu," in the same sense that WW I was "just a war".

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