Back Of The Envelope COVID19

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Thinking out loud ...

Here are some facts:

  • The mortality rate of COVID-19 is about 1%;
  • Roughly the number of cases trebles every 5 days ...
    • ... or roughly by 10 every 10 days.
  • Time from infection to death is about 3 weeks, say 20 days.

So ...

As I write this, on March 12, there have been 10 deaths in the UK. At a mortality of 1%, that means around 1000 people had the virus 20 days ago.

But it's multiplied by 10 twice since then. On that basis ...

The current number of infected people in the UK right now, detected and undetected, is about 100K.


You can play with the numbers to get upper and lower bounds if you like, and I strongly advise you to do so, but the trend is unmistakable.

The number of people infected and infectious is about ten thousand times the number of recorded deaths.

Slightly more nuanced ...

When I posted this, a number of people got in touch (out of band) to point out that it cannot remain exponential because we'll run out of people. That's true. Eventually the R0 drops because each infected person meets fewer uninfected people.

But when you run the sums you find that the numbers start to deviate significantly from the exponential when we have overwhelmingly many cases.

Overwhelmingly many.

So the point stands:


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