
Ask HN: Where does the herd immunity number comes from? - econcon
Some claim it&#x27;s 40% while some says it&#x27;s 70% and some claim it to be 60%. Where does this number comes from?
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philcowans
I'm not an expert, and I'm sure it's more complex than this in practice, but
if each infected individual transmits the infection to n others, and you
assume that a fraction f of the population are immune, you want

(1-f)*n < 1

to avoid exponential growth, so

f = 1 - 1/n

Your examples cover a range of 1.7 to 3.3 as estimates for n, which is roughly
aligned with what I've heard reported.

