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Half of the U.S. population has been infected with COVID-19, including 75% of children and adolescents, CDC officials announced on Tuesday.

From December 2021 to February 2022, during the Omicron wave in the U.S., overall seroprevalence increased from 33.5% to 57.7%, reported Kristie Clarke, MD, of the CDC's COVID-19 Emergency Response Team, and colleagues in an early edition of the .

Over this same time period, seroprevalence increased from 45.6% to 74.2% among adolescents ages 12 to 17, and from 44.2% to 75.2% among children ages 11 and younger.

"We definitely expected to see an increase. We didn't expect it to increase quite this much, but we follow the data," said Clarke at a media briefing.

She noted that they examined data on the proportion of the population who developed antibodies to the nucleocapsid protein, which only appear after infection, not vaccination.

When asked why seroprevalence appeared to be so much higher in children and adolescents versus older adults (for whom seroprevalence was only 33%), Clarke pointed out that younger populations have the lowest vaccination rates, at 28% for kids ages 5 to 11 versus 90% for adults ages 65 and up.

However, she cautioned against the assumption that infection-induced immunity was sufficient protection from reinfection, noting that "protection from infection and vaccination wane over time" and that vaccination gives kids "broad protection" against future infection.

"No known threshold of the population [with immunity] ... will stop community transmission of SARS-CoV-2," Clarke said. "Reinfection happens."

Indeed, she explained that the study did not look at the level of antibodies from prior infection that provides protection against reinfection or severe disease, noting that the test "detects a very low level of anti-nucleocapsid antibodies" and "gives a yes/no answer" about infection.

"It's a good measure of what proportion of the population has been previously infected, but we don't know the level of antibodies each person has with each type of test," she added.

Clarke said that they are moving to include a quantitative antibody test in the future, and they will continue to conduct more research on how long it takes for protection to wane. "Infection after vaccination may be less likely to result in detectable antibodies," she noted.

CDC Director Rochelle Walensky, MD, provided some context to the findings, explaining that all sequenced infections in the U.S. are Omicron, with subvariant BA.2 accounting for 68% of circulating viruses, and the BA.2.121 subvariant, which was originally identified in upstate New York, comprising 30% of sequences identified.

She said that they believe BA.2.121 has about a 25% transmission advantage over BA.2, but there is no evidence that it produces more severe disease.

Clarke noted that preprint data scheduled to be released soon found that the infection-to-case ratio is now the highest it's been throughout the pandemic (over three estimated infections per case), though it varies by region. A prior study found an "increased ratio of infections per reported case in the pediatric population," she added.

Walensky pointed to the "mixed trends" that the CDC has continued to see throughout the pandemic, where deaths are declining (down 18% vs the prior week), but hospitalizations are trending upwards -- about 9% higher than a week ago -- while cases have increased 25% since last week to about 44,000 a day.

"We continue to believe those who are vaccinated and those who are boosted continue to have strong protection against severe disease," she said.

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