Ask HN: Where does the herd immunity number comes from?
Some claim it's 40% while some says it's 70% and some claim it to be 60%. Where does this number comes from?
(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.