A medical assistant in New York took a blood sample for a test for antibodies to coronavirus in January. Credit … James Estrin / New York Times
The common belief that almost everyone in America seems to have acquired the Omicron version last winter may not be far from the truth. By February 2022, nearly 60 percent of the population was infected with the coronavirus, nearly twice as many as in December 2021, according to data released Tuesday by the Centers for Disease Control and Prevention.
“By February 2022, the evidence for previous Covid-19 infections had increased significantly among each age group,” said Dr. Christie Clark, the agency’s researcher who led the study.
Infections increased most sharply during the Omicron jump among children and adolescents, perhaps because many people in these age groups had not yet been vaccinated. The increase is least among adults aged 65 and over who have the highest vaccination rates and are more likely to take precautions.
A new study shows that three in four children and adolescents in the United States were infected with the coronavirus by February 2022, compared with one-third of adults.
While some studies have shown that previous infection offers a weaker shield against the virus than vaccines, the antibodies obtained must provide a reasonable degree of protection against severe disease, at least in the short term.
“We still don’t know how long the immunity caused by the infection will last,” Dr. Clark said.
Increasing the immunity of the entire population may explain why the new surge, which has spread to China and many countries in Europe, has been silenced in the United States so far.
The findings may offer some consolation to parents who have been looking forward to the vaccine being approved for the youngest children. Many of these children now seem to have acquired at least some immunity.
However, Dr. Clark called on parents to immunize eligible children as soon as regulators allow a vaccine for them, regardless of previous infection. She noted that when children are hospitalized with coronavirus, up to 30 percent of them may need intensive care.
Many of these children have other medical conditions. But up to 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 parent, I would absolutely recommend that children be vaccinated, even if they have been infected,” said Dr. Clark.
Confirmed coronavirus cases are on the rise again in the United States, especially in the northeast, but so far the increase in hospitalizations has been minimal and deaths are still declining.
Even among those hospitalized, “we see less oxygen use, less in the intensive care unit and, fortunately, we haven’t seen an increase in related deaths,” said the CDC director, Dr. Rochelle Valensky. “We hope that the positive trends will continue.”
CDC researchers began assessing human antibody levels in 10 places at the start of the pandemic and have since expanded their efforts to all 50 states, the District of Columbia and Puerto Rico. Researchers use a sensitive enough test to identify previously infected people at least a year or two after exposure.
The researchers analyzed blood samples collected from September 2021 to February 2022, looking for antibodies against the virus; they then analyze data by age, gender and geographical location. The researchers looked for a specific type of antibody that is produced after an infection, but not in people who have simply been vaccinated.
Between September 2021 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, increasing by nearly 25 points by February 2022.
The percentage of antibody samples increased to about 75 percent from about 45 percent in both children 11 years of age and younger and adolescents 12 to 17 years of age.
By February 2022, approximately 64 percent of adults aged 18 to 49, about half of those aged 50 to 64, and about a third of adults were infected with the virus, according to the study.
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