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 scientist. (That includes many people who have never taken a Covid test.) About 18 percent have received at least one vaccine shot. There is some overlap between these two groups, which means that about 40 percent of Americans now have some protection from Covid.
Had these people been exposed to the virus a year ago, they could have become infected — and then spread Covid to others. Today, many are protected.
“This level of population immunity slows…