The common perception that nearly everyone in America seemed to have acquired the Omicron variant last winter may not have been far from the truth. By February 2022, nearly 60 percent of the population had been infected with the coronavirus, almost double the proportion seen in December 2021, according to data released on Tuesday by the Centers for Disease Control and Prevention.
“By February 2022, evidence of previous Covid-19 infections substantially increased among every age group,” Dr. Kristie Clarke, the agency researcher who led the study, said at a news briefing.
Infections rose most sharply during the Omicron surge among children and adolescents, perhaps because many people in those age groups were still unvaccinated. The increase was smallest among adults 65 or older, who have the highest rate of vaccination and may be the most likely to take precautions.
The new research suggests that three out of four children and adolescents in the United States had been infected with the coronavirus by February 2022, compared with one-third of older adults.
While some studies suggest that prior infection offers a weaker shield against the virus than vaccines do, the resulting antibodies should provide a reasonable degree of protection against severe illness, at least in the short term.
“We still do not know how long infection-induced immunity will last,” Dr. Clarke said.
The gains in population-wide immunity may explain why the new surge that is roaring through China and many countries in Europe has been muted in the United States so far.
The findings may offer some comfort to parents who have been waiting anxiously for a vaccine to be authorized for the youngest children. Many of those children now seem to have acquired at least some immunity.
Even so, Dr. Clarke urged parents to immunize children who qualify as soon as regulators authorize a vaccine for them, regardless of any prior infection. She noted that when children are hospitalized with the coronavirus, up to 30 percent of them may need intensive care.
Many of those children also have other medical conditions. But as many as 70 percent of cases of multisystem inflammatory disease, a rare consequence of coronavirus infection, occur in children who were otherwise healthy.
“As a pediatrician and a parent, I would absolutely endorse that children get vaccinated, even if they have been infected,” Dr. Clarke said.
Confirmed coronavirus cases are rising again in the United States, particularly in the Northeast, but so far the rise in hospitalizations has been minimal, and deaths are still dropping.
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 P. Walensky. “We are hopeful that positive trends will continue.”
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 use a test sensitive enough to identify previously infected people for at least a year or two after their exposure.
The researchers analyzed blood samples collected from September 2021 to February 2022, looking for antibodies to the virus; then they parsed the data by age, sex and geographic location. The investigators looked specifically for a type of antibody that is produced after infection, but not in people who have merely been vaccinated.
Between September 2021 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 to about 75 percent from about 45 percent among both children aged 11 years and younger, and adolescents aged 12 to 17.
By February 2022, roughly 64 percent of adults 18 to 49, about half those 50 to 64, and about one-third of older adults had been infected with the virus, according to the study.