But prior infection does not guarantee protection from the virus, officials said, and Americans should still get vaccinated and boosted.
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.
Dr. Jha and other officials warned against complacency, and urged Americans to continue receiving vaccinations and booster shots, saying that antibodies from prior infections did not guarantee protection from the virus.
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.
While the focus is often on preventing the health care system from buckling under a surge, “we should also be concerned that our health care system will be overwhelmed by the ongoing health care needs of a population with long Covid,” said Zoë McLaren, a health policy expert at the University of Maryland, Baltimore County.
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.”
Although cases are once again on the upswing, particularly in the Northeast, the rise in hospitalizations has been minimal, and deaths are still dropping. According to the agency’s most recent criteria, more than 98 percent of Americans live in communities with a low or medium level of risk.
Even among those who are hospitalized, “we’re seeing less oxygen use, less I.C.U. stays and we haven’t, fortunately, seen any increase in deaths associated with them,” said the C.D.C.’s director, Dr. Rochelle Walensky. “We are hopeful that positive trends will continue.”
The country has recorded about a five-fold drop in P.C.R. testing for the virus since the Omicron peak, and so tracking new cases has become difficult. But the reported count is far less, about 70-fold lower, said Dr. Walensky, reflecting “a true and reliable drop in our overall cases.”
New subvariants of Omicron, called BA.2 and BA.2.12.1, have supplanted the previous iteration, BA.1, which began circulating in the country in late November and sent cases soaring to record highs in a matter of weeks.
“Of course, even more have been infected now, because BA.2 will have infected some who avoided it thus far,” Dr. Hanage said.
By February, three of four children and adolescents in the country had already been infected with the virus, compared with one-third of older adults, according to the new study.
That so many children are carrying antibodies may offer comfort to parents of those aged 5 and under, who do not qualify for vaccination, since many may have acquired at least some immunity through infection.
But Dr. Clarke urged parents to immunize children who qualify as soon as regulators approve a vaccine for them, regardless of their prior infection. Among children who are hospitalized with the virus, up to 30 percent may need intensive care, she noted.
Although many of those children also have other medical conditions, about 70 percent of cases of multisystem inflammatory disease, a rare consequence of Covid-19 infection, occur in otherwise healthy children.
“As a pediatrician and a parent, I would absolutely endorse the children get vaccinated, even if they have been infected,” Dr. Clarke said.
Some experts said they were concerned about long-term consequences, even in children who have mild symptoms.
“Given the very high proportion of infection in kids and adults that happened earlier this year, I worry about the rise in long Covid cases as a result,” said Akiko Iwasaki, an immunologist at Yale University who is studying the condition.
To measure of the percentage of the population infected with the virus, the study relied on the presence of antibodies produced in response to an infection.
C.D.C. researchers began assessing antibody levels in people at 10 sites early in the pandemic, and have since expanded that effort to all 50 states, the District of Columbia and Puerto Rico. The investigators used a test sensitive enough to identify previously infected people for at least one to two years after exposure.
The researchers analyzed blood samples collected from September 2021 to February 2022 for antibodies to the virus, and then parsed the data by age, sex and geographical location. The investigators looked specifically for a type of antibody produced after infection but not after vaccination.
Between September and December 2021, the prevalence of antibodies in the samples steadily increased by one to two percentage points every four weeks. But it jumped sharply after December, increasing by nearly 25 points by February 2022.
The percentage of samples with antibodies rose from about 45 percent among children aged 11 years and younger, and among adolescents aged 12 to 17 years, to about 75 percent in both age groups.
By February 2022, roughly 64 percent of adults aged 18 to 49 years, about 50 percent of those aged 50 to 64 years and about 33 percent of older adults had been infected, according to the study.
The C.D.C. may have underestimated of the number of Americans who have been infected, Dr. McLaren said.
Despite the record high cases during the Omicron surge, the reported statistics may not have captured all infections, because some people have few to no symptoms, may not have opted for testing, or may have tested themselves at home.
According to one upcoming C.D.C. study, there may be more than three infections for each reported case, Dr. Clarke said.
Noah Weiland contributed reporting from Washington.