Thread by @MettaBhavana1

With Covid-19, the R0 [average number of people each infected person spreads the disease to...] is not well known (and depends on social factors), but Imperial College puts it between 1.5 and 3.5 and the WHO at between 1.4 and 2.5. Let’s take the highest estimate. 1.

If having the disease once makes you immune forever, then 100-(100/3.5)=71%; herd immunity would be achieved when 71% of the population has had it. 2.

Of course, that assumes perfect immunity. If — as with flu — that’s not the case, the maths is very different. In fact, if the percentage who are still vulnerable after vaccination or exposure is greater than the percentage required for herd immunity, then it becomes impossible

to achieve herd immunity, even if 100% population is exposed or vaccinated. At the moment it’s not clear whether Covid-19 will become seasonal, like flu, or will be a once-in-a-lifetime illness like measles. It makes a huge difference to whether herd immunity will work or not. 4

Can we build a herd immunity to Covid-19?

TOM CHIVERS
Friday, 13 March 2020

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