Public discussion of “herd immunity” often treats it like an on-off switch: When the U.S. reaches herd immunity, the crisis will be over; until then, the country has little immunity from Covid-19.
But that’s not right.
Herd immunity is more like a light dimmer. The more people develop immunity — either from having been infected or from being vaccinated — the less easily the virus will spread.
Nearly 30 percent of Americans have now had the virus, according to Youyang Gu, a data…
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