Mathematically, the herd immunity threshold is defined by 1 – 1/R0
the point i'm trying to make is that i think everyone is familiar with r
o and how that isn't the same thing as the effective (or instaneous) rate of transmission r
eff (or r
t), which is the rate we actually observe in the wild.
the same thing applies to herd immunity. the actual observed level at which a herd is immune is a function of r
eff, not r
o. so, like, if you're in a herd where everyone is modifying their behaviors to avoid transmission, or where theaters and sporting events are closed or whatever, then you will observe herd immunity at the level based on the r
eff that all those interventions produce.
until everyone stops modifying their behavior, of course.