Sixty percent of Americans, including 75 percent of children, had been infected with the coronavirus by February, federal health officials reported Tuesday, another notable milestone in a pandemic that continues to confound expectations.
The highly contagious Omicron variant was responsible for much of the toll. In December 2021, when the variant began to spread, only half of people had antibodies indicating prior infection, according to new research from the Centers for Disease Control and Prevention.
While the numbers surprised many Americans, some scientists said they expected the numbers to be even higher, given the contagious strains that have swept through the nation in the past two years.
There may be good news in the data, some experts said. A gain in population-wide immunity can offer at least a partial bulwark against future waves. And the trend may explain why the surge now raging in 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 illnesses or deaths related to infections. “We will see less and less severe disease, and more and more of a shift to clinically mild disease,” said Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai in New York.
“It will be increasingly difficult for the virus to do serious damage,” he added.
Administration officials also believe the data portends a new phase of the pandemic in which infections may be common at times but cause less damage.
At a news conference on Tuesday, Dr. Ashish Jha, the White House’s new Covid coordinator, said stopping infections “wasn’t even a policy goal. The goal of our policy should be: obviously minimize infections wherever possible, but make sure people don’t get seriously ill.”
The average number of new confirmed cases per 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 increased by more than 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 boosters, saying antibodies from previous infections did not guarantee protection against the virus.
During the Omicron surge, infections rose the most among children and teens, according to the new research. Previous infections increased least among adults 65 and older, who have the highest vaccination rates and are most likely to take precautions.
“Evidence of prior Covid-19 infections increased substantially among all age groups,” Dr. Kristie Clarke, an agency investigator who led the new study, said at a news conference Tuesday.
The widespread infection poses a worrying prospect: a potential rise in prolonged Covid cases, a little-known constellation of lingering symptoms.
Up to 30 percent of people infected with the coronavirus may have persistent symptoms, including worrying changes in the brain and heart. Vaccination is thought to reduce the chances of prolonged covid, though by how much is unclear.
“The long-term impacts on health care are unclear, but certainly worth taking very seriously, as a fraction of people will struggle for a long time with the consequences,” said Bill Hanage, an epidemiologist at the Harvard TH Chan School of Public. Health.
Even a very small percentage of infected or vaccinated people who develop Covid would translate into millions across the country.
While the focus is often on keeping the health care system from collapsing in the face of a surge, “we should also be concerned about our health care system being overwhelmed by the ongoing health care needs of a population with prolonged Covid,” said Zoë McLaren, a health policy expert at the University of Maryland, Baltimore County.
There are still tens of millions of Americans without immunity to the virus, and they remain vulnerable to both short-term and long-term consequences of infection, said Dr. Tom Inglesby, director of the Center for Health Security at Johns Hopkins. Bloomberg. School of Public Health.
“Betting you’re at 60 percent is a big bet,” he said. “For anyone who hasn’t been vaccinated and boosted, I would take this new data as a direct message to do so or hope the virus catches up with you if it hasn’t already.”
Although cases are on the rise again, particularly in the Northeast, the increase in hospitalizations has been minimal and deaths continue to decline. According to the agency’s most recent criteria, more than 98 percent of Americans live in communities with a low or medium risk level.
Even among those who are hospitalized, “we’re seeing less oxygen use, fewer ICU stays, and fortunately we haven’t seen any increase in deaths associated with them,” said CDC Director Dr. Rochelle Walensky. “We are hopeful that the positive trends will continue.”
The country has seen a five-fold drop in PCR tests for the virus since Omicron’s spike, so tracking new cases has become difficult. But the reported count is much lower, about 70 times lower, Dr. Walensky said, reflecting “a real and reliable drop in our cases overall.”
The new Omicron sub-variants, 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 to record levels in a matter of weeks. .
“Of course, even more have been infected now, because BA.2 will have infected some who have avoided it so far,” said Dr. Hanage.
By February, three out of four children and adolescents in the country had already been infected with the virus, compared to a third of older adults, according to the new study.
That so many children carry antibodies may offer comfort to parents of children under 5 who do not qualify for vaccination, as many may have acquired at least some immunity through infection.
But Dr. Clarke urged parents to vaccinate eligible children as soon as regulators approve a vaccine for them, regardless of their previous infection. Among children hospitalized with the virus, up to 30 percent may need intensive care, he 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 support children getting vaccinated, even if they have been infected,” said Dr. Clarke.
Some experts said they were concerned about the long-term consequences, even for children with mild symptoms.
“Given the very high proportion of infection in children and adults that occurred earlier this year, I am concerned about the increase in long-term cases of Covid as a result,” said Akiko Iwasaki, an immunologist at Yale University who is studying the condition.
To measure the percentage of the population infected with the virus, the study relied on the presence of antibodies produced in response to an infection.
CDC researchers began testing people for antibody levels 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 researchers used a test sensitive enough to identify previously infected people for at least one to two years after exposure.
The researchers tested blood samples collected between September 2021 and February 2022 for antibodies to the virus and then analyzed the data by age, gender and geographic location. The researchers specifically looked for a type of antibody produced after infection but not after vaccination.
Between September and December 2021, the prevalence of antibodies in the samples increased steadily by one to two percentage points every four weeks. But it jumped sharply after December, rising almost 25 points in February 2022.
The percentage of samples with antibodies increased from about 45 percent among children 11 years old and younger, and among adolescents 12 to 17 years old, to about 75 percent in both age groups.
By February 2022, about 64 percent of adults ages 18 to 49, about 50 percent of those ages 50 to 64, and about 33 percent of older adults had been infected, according to the study.
Despite the record number of cases during the Omicron surge, reported statistics may not have captured all infections, because some people have few or no symptoms, may not have chosen to be tested, or may have tested at home. .
According to an upcoming CDC study, there may be more than three infections for every reported case, Dr. Clarke said.
Noah Weiland contributed reporting from Washington.