Thread by @CiaraMcMillan4

A #COVID stats guide:
What's worse, a disease with higher transmissibility, or one with higher fatality rate?

Intuitively, it's "obvious" that the less-fatal, more-transmissible disease is preferable, so we shouldn't be scared of the Delta variant, right?

Wrong
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A lot of people make this mistake, which is fine. Statistics can be counterintuitive, but here's a worked example to help you make sense of it.

Keeping the numbers simple: imagine an asymptomatic disease with reproductive rate (R) of 1 and a fatality rate of 1%.

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#COVID

With these numbers, you would expect every 10,000 people to infect another 10,000 people (so, no growth), and in each generation of 10,000 people you would expect 100 deaths.

In 10 generations of disease spread:
100,000 people are infected, and 1,000 die

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#COVID

So, let's make a variant of our disease that is 50% more fatal. The maths are pretty simple here, the same number get infected, but 50% more die in each generation of spread.

So:
100,000 people are infected, and 1,500 die

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#COVID

Now, let's make a different variant that has the original fatality rate of 1%, but we increase the transmissibility rate by 50%.

Sounds better, yes? Intuitively, you'd rather a more transmissible disease than a more deadly one? Right?

Let's run the numbers:

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#COVID

For our 50% more transmissible disease, the first number of infected people grows geometrically.

Gen 1: 10,000 infected, 1% die = 100 deaths
Gen 2: 15,000 infected, 1% die = 150
Gen 3: 22,500 infected, 1% die = 225
Gen 4: 33, 750 infected, 1% die = 337

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#COVID

After 10 generations of disease spread, a total of 1.1 million people are infected, with >11,000 deaths. Compared to the 1,500 deaths you would see from the "more deadly" disease.

It's like compound interest, only for death.

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#COVID

So, when people dismiss the Delta variant as being a harmless disease with a low fatality rate, that just happens to be more transmissible, it seems "intuitively" reasonable to do so.

But it's wrong. Deadly wrong.

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#COVID

Even asymptomatic people spread the disease. The new variants are more transmissible, and as we've seen above, this means more people will die.

Get the vaccine.

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#COVID

It “feels” wrong, so people don’t want to believe it. Here’s an example: “something can be extremely contagious like the common cold, or something not contagious can be extremely dangerous like Rabies”

Reader, the common cold kills MANY more people than Rabies.

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#COVID

Here’s another:
“If you had to choose common cold (high transmission rates) or say Ebola (50 per cent death rate but hard to catch), you’d choose the former”.

Great example! Sounds intuitively right, but it’s dead wrong.

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Total worldwide deaths from Ebola since 1976 are approx 15K

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Common cold deaths are notoriously hard to estimate, but just one of the common Respiratory syncytial virus (RSV) kills 14K elderly (>65y) per year in the US alone, and is the leading cause of respiratory mortality in infants (estimated at over 66K/yr; Nair et al, 2010).

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Why does any of this matter? One simple reason: the newer variants of #COVID are more contagious. More people will be infected. People will shrug and say “so what, there’s such a low risk of dying from it”. That’s missing the point…

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Hopefully the figures above help you understand, when a low per-person risk is combined with such a high transmission rate, you’re looking at a lot of deaths.

Stay safe.
Get the #CovidVaccine

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