Sixty percent of Americans, including 75 percent of children, had been infected with the coronavirus by February, federal health officials reported on Tuesday — another remarkable milestone in a pandemic that continues to confound expectations.
The highly contagious Omicron variant was responsible for much of the toll. In December 2021, as the variant began spreading, only half as many people had antibodies indicating prior infection, according to new research from the Centers for Disease Control and Prevention.
While the numbers came as a shock to many Americans, some scientists said they had expected the figures to be even higher, given the contagious variants that have marched through the nation over the past two years.
There may be good news in the data, some experts said. A gain in population-wide immunity may offer at least a partial bulwark against future waves. And the trend may explain why the surge that is now roaring through China and many countries in Europe has been muted in the United States.
A high percentage of previous infections may also mean that there are now fewer cases of life-threatening illness or death relative to infections. “We will see less and less severe disease, and more and more a shift toward clinically mild disease,” said Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai in New York.
“It will be more and more difficult for the virus to do serious damage,” he added.
Administration officials, too, believe that the data augur a new phase of the pandemic in which infections may be common at times but cause less harm.
At a news briefing on Tuesday, Dr. Ashish Jha, the White House’s new Covid coordinator, said that stopping infections was “not even a policy goal. The goal of our policy should be: obviously, minimize infections whenever possible, but to make sure people don’t get seriously ill.”
The average number of confirmed new cases a day in the United States — more than 49,000 as of Monday, according to a New York Times database — is comparable to levels last seen in late July, even as cases have risen by over 50 percent over the past two weeks, a trend infectious disease experts have attributed to new Omicron subvariants.
During the Omicron surge, infections rose most sharply among children and adolescents, according to the new research. Prior infections increased least among adults aged 65 and older, who have the highest rates of vaccination and may be most likely to take precautions.
“Evidence of previous Covid-19 infections substantially increased among every age group,” Dr. Kristie Clarke, the agency researcher who led the new study, said at a news briefing on Tuesday.
Widespread infection raises a troubling prospect: a potential increase in cases of long Covid, a poorly understood constellation of lingering symptoms.
Up to 30 percent of people infected with the coronavirus may have persistent symptoms, including worrisome changes to the brain and heart. Vaccination is thought to lower the odds of long Covid, although it is unclear by how much.
“The long-term impacts on health care are not clear but certainly worth taking very seriously, as a fraction of people will be struggling for a long time with the consequences,” said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.
Even a very small percentage of infected or vaccinated people who develop Covid would translate to millions nationwide.
There are still tens of millions of Americans with no immunity to the virus, and they remain vulnerable to both the short- and long-term consequences of infection, said Dr. Tom Inglesby, director of the Center for Health Security at the Johns Hopkins Bloomberg School of Public Health.
“Betting that you are in the 60 percent is a big gamble,” he said. “For anyone who’s not been vaccinated and boosted, I would take this new data as a direct message to get that done or expect that the virus is likely to catch up to you if it hasn’t already.”
Noah Weiland contributed reporting from Washington.