Work out herd immunity threshold instantly with clear inputs, formula shown and shareable results.
Applies the classic threshold of one minus the reciprocal of R0 and divides it by vaccine efficacy to give the coverage required.
Herd immunity threshold
HIT = 1 - 1/R0; required vaccination coverage = HIT / vaccine efficacy
With R0 around 12 to 18 the threshold is 92 to 95%, which leaves almost no room for imperfect efficacy or uptake.
It assumes random mixing; clustered susceptibility means outbreaks can occur even above the nominal threshold.