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Covid-19 5/7: Fighting Limbo

Right. Also:

" Shame on anyone who treats herd immunity as a boolean, rather than something that doesn’t do anything until you hit a threshold. "

This argument cuts both ways. "Herd immunity" is also not some magically threshold, after which *no one* gets infected. There's a term for it: overshoot.

From this piece by Carl Bergstrom and Natalie Dean, professors of biology and biostatistics: "Once enough immunity has been built in the population, each person will infect fewer than one other person, so a new epidemic cannot start afresh. But an epidemic that is already underway will continue to spread. If 100,000 people are infectious at the peak and they each infect 0.9 people, that’s still 90,000 new infections, and more after that. A runaway train doesn’t stop the instant the track begins to slope uphill, and a rapidly spreading virus doesn’t stop right when herd immunity is attained.

If the pandemic went uncontrolled in the United States, it could continue for months after herd immunity was reached, infecting many more millions in the process.

By the time the epidemic ended, a very large proportion of the population would have been infected — far above our expected herd immunity threshold of around two-thirds. These additional infections are what epidemiologists refer to as “overshoot.”"