Deadly virus surge now helping to create herd immunity in U.S.

LOS ANGELES — As coronavirus cases plummet nationwide and vaccinations total 1.7 million Americans a day and rising, health experts are increasingly striking a new tone in their pandemic assessments: optimism.

“I could be wrong, but I don’t think we’re going to see a big fourth surge,” said Dr. Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia. “I think we’ve seen the worst of it.”

Many epidemiologists and other scientists, while still cautious, say they feel increasingly hopeful that the rest of 2021 will not replay the nightmare of last year.

The arrival of spring will likely aid the ongoing precipitous drop in coronavirus cases, as warmer weather allows people to spend more time outdoors and creates a less hospitable environment for the virus, experts say.

But the biggest factor, paradoxically, is something the nation spent the last year trying to prevent.

While 12% of Americans have received at least one dose of the COVID-19 vaccine, far more people — approximately 35% of the nation’s population — have already been infected with the coronavirus, Dr. Offit estimated. Studies have found that people who survive COVID-19 have immunity for several months, though it likely lasts even longer.

University of California, San Francisco epidemiologist Dr. George Rutherford said one of the reasons why cases are dropping so fast in California “is because of naturally acquired immunity, mostly in Southern California.” He estimated that 50% of Los Angeles County residents have been infected with the virus at some point.

“We’re really talking something starting to sound and look like herd immunity — although that true herd immunity is a ways off in the future,” Dr. Rutherford said recently.

Herd immunity is reached when so many people have immunity that a virus cannot find new hosts and stops spreading, resulting in communitywide protection. Scientists believe that in the case of the coronavirus, the threshold could be as high as 90%. The United States has not met this threshold but each step toward it slows transmission, experts say.

The effects may be greatest in places that endured the worst COVID-19 surges, including Los Angeles. After a horrific autumn and winter wave that has killed more than 12,000 people, an estimated 33% to 55% of county residents have already been infected with the coronavirus, according toUSC researchers.