The equation used to calculate herd immunity is very simple, so simple that even I can understand it and you can too: threshold = 1 - 1/R0. He prints equation. Eyes roll back in heads. But no, wait, look how easy this is. If the reproduction number is three, meaning three secondary cases infected by each primary case, you need nothing more than grade-school fractions to work it out: 1 minus 1/3 equals what? Two thirds, right? So, in that case, the threshold for herd immunity is two thirds: 67 percent of the population.
