Thread by @TonyMcKelvie

By and for Dummies...
Herd Immunity is the proportion of a given population that requires to be immune from a given illness in order that the illness is restricted from spreading.
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We're familiar with vaccination programmes which seek to ensure that a sufficient proportion (the threshold) of the population is immunised against a given disease to prevent outbreaks occurring.
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The threshold is determined by the Reproduction Number (Ro) of the illness - which is the number of people one infected person will go on to infect in turn.
The formula is (Ro-1)/Ro
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For Coronavirus (CoVid-19) the Ro = 2.3 according to WHO estimates.
That is, it has been observed that each new case of CoVid-19 causes on average 2.3 further new cases.
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Applying the formula, we can calculate the immunity threshold for the illness as:
(Ro-1)/Ro

(2.3-1)/2.3

= 56.5%

This indicates that when 56.5% of people have acquired immunity from the illness, the illness will be restricted from spreading further.

It is important to note that there is no vaccine for CoVid-19, and until/unless one is developed, there can be no immunisation programme.
Immunity is therefore acquired by individual exposure to the virus, our body's own immune system then generating the response to it.
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WHO observations suggest that of those people exposed to the virus, around 17% will become infected, of which half will develop symptoms, the other half being 'asymptomatic' showing no or little apparent illness.
Let's say that 9% develop symptomatic illness.
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Now let's apply the Herd Immunity Threshold:
In order for (say) UK to acquire herd immunity from CoVid-19, some 56.5% of the population will require to have been exposed to the virus.
Of those 56.5%, around 9% will develop symptomatic illness.
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In round numbers:

UK population = 66.5 Million

x 56.5% Immunity Threshold = 37.5 Million people

x 9% symptomatic illness rate = 3.4 Million

That is, in order to acquire herd immunity, around 3.4 Million people will develop symptomatic illness.
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The data from Italy suggests that of those who develop symptomatic illness, around 10% require hospital treatment, and the average stay in hospital is 10 days.

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If you are currently planning a CoVid-19 strategy for the UK based on Herd Immunity, you need to have the resources to provide critical care in hospital for around 340,000 cases of duration 10 days over the coming months.

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Because it looks like 10% of new cases require hospital treatment, and each treatment lasts 10 days, the crude arithmetic suggests that you need a critical care bed for every new case.
That is, if there are 100 new cases each day, you need capacity of 100 beds.
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Because the number of new cases doubles every 4 days approx, the numbers become quite large quite quickly.

Eg In Italy 2 weeks ago there were 238 new cases. Yesterday there were 2651 new cases.

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If you are planning a herd immunity programme in the UK, you need enough critical care beds to match the daily new cases load.

I would expect, in that situation, that great efforts are being made to create enough critical care beds to match the estimated new cases 'peak'.
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As noted at the start of this thread, this thread is strictly by and for Dummies.

If the assumptions are incorrect or the arithmetic is wrong please let me know asap and I'll edit/amend.

The End