At what point does a country achieve herd immunity? What portion of the population must acquire resistance to the coronavirus, either through infection or vaccination, in order for the disease to fade away and life to return to normal?

Since the start of the pandemic, the figure that many epidemiologists have offered has been 60 to 70 percent. That range is still cited by the World Health Organization and is often repeated during discussions of the future course of the disease.

Although it is impossible to know with certainty what the limit will be until we reach it and transmission stops, having a good estimate is important: It gives Americans a sense of when we can hope to breathe freely again.

Recently, a figure to whom millions of Americans look for guidance — Dr. Anthony S. Fauci, an adviser to both the Trump administration and the incoming Biden administration — has begun incrementally raising his herd-immunity estimate.

Ever since the race to develop a coronavirus vaccine began last spring, upbeat announcements were stalked by ominous polls: No matter how encouraging the news, growing numbers of people said they would refuse to get the shot.

The time frame was dangerously accelerated, many people warned. The vaccine was a scam from Big Pharma, others said. A political ploy by the Trump administration, many Democrats charged. The internet pulsed with apocalyptic predictions from longtime vaccine opponents, who decried the new shot as the epitome of every concern they’d ever put forth.