Sixty % of People, together with 75 % of youngsters, had been contaminated with the coronavirus by February, federal well being officers reported on Tuesday — one other exceptional milestone in a pandemic that continues to confound expectations.

The extremely contagious Omicron variant was accountable for a lot of the toll. In December 2021, because the variant started spreading, solely half as many individuals had antibodies indicating prior an infection, in keeping with new analysis from the Facilities for Illness Management and Prevention.

Whereas the numbers got here as a shock to many People, some scientists stated they’d anticipated the figures to be even greater, given the contagious variants which have marched via the nation over the previous two years.

There could also be excellent news within the knowledge, some consultants stated. A acquire in population-wide immunity could provide not less than a partial bulwark towards future waves. And the pattern could clarify why the surge that’s now roaring via China and lots of nations in Europe has been muted in the US.

A excessive proportion of earlier infections can also imply that there are actually fewer instances of life-threatening sickness or demise relative to infections. “We’ll see much less and fewer extreme illness, and an increasing number of a shift towards clinically delicate illness,” stated Florian Krammer, an immunologist on the Icahn College of Drugs at Mount Sinai in New York.

“Will probably be an increasing number of tough for the virus to do severe harm,” he added.

Administration officers, too, imagine that the info augur a brand new section of the pandemic by which infections could also be widespread at occasions however trigger much less hurt.

At a information briefing on Tuesday, Dr. Ashish Jha, the White Home’s new Covid coordinator, stated that stopping infections was “not even a coverage purpose. The purpose of our coverage ought to be: clearly, reduce infections every time attainable, however to ensure individuals don’t get critically sick.”

The typical variety of confirmed new instances a day in the US — greater than 49,000 as of Monday, in keeping with a New York Occasions database — is akin to ranges final seen in late July, at the same time as instances have risen by over 50 % over the previous two weeks, a pattern infectious illness consultants have attributed to new Omicron subvariants.

Dr. Jha and different officers warned towards complacency, and urged People to proceed receiving vaccinations and booster pictures, saying that antibodies from prior infections didn’t assure safety from the virus.

In the course of the Omicron surge, infections rose most sharply amongst youngsters and adolescents, in keeping with the brand new analysis. Prior infections elevated least amongst adults aged 65 and older, who’ve the best charges of vaccination and could also be most definitely to take precautions.

“Proof of earlier Covid-19 infections considerably elevated amongst all ages group,” Dr. Kristie Clarke, the company researcher who led the brand new research, stated at a information briefing on Tuesday.

Widespread an infection raises a troubling prospect: a possible enhance in instances of lengthy Covid, a poorly understood constellation of lingering signs.

As much as 30 % of individuals contaminated with the coronavirus could have persistent signs, together with worrisome modifications to the mind and coronary heart. Vaccination is believed to decrease the chances of lengthy Covid, though it’s unclear by how a lot.

“The long-term impacts on well being care aren’t clear however actually price taking very critically, as a fraction of individuals can be struggling for a very long time with the implications,” stated Invoice Hanage, an epidemiologist on the Harvard T.H. Chan College of Public Well being.

Even a really small proportion of contaminated or vaccinated individuals who develop lengthy Covid would translate to hundreds of thousands nationwide.

Whereas the main focus is usually on stopping the well being care system from buckling beneath a surge, “we also needs to be involved that our well being care system can be overwhelmed by the continued well being care wants of a inhabitants with lengthy Covid,” stated Zoë McLaren, a well being coverage skilled on the College of Maryland, Baltimore County.

There are nonetheless tens of hundreds of thousands of People with no immunity to the virus, they usually stay weak to each the short- and long-term penalties of an infection, stated Dr. Tom Inglesby, director of the Heart for Well being Safety on the Johns Hopkins Bloomberg College of Public Well being.

“Betting that you’re within the 60 % is an enormous gamble,” he stated. “For anybody who’s not been vaccinated and boosted, I might take this new knowledge as a direct message to get that carried out or anticipate that the virus is prone to catch as much as you if it hasn’t already.”

Though instances are as soon as once more on the upswing, significantly within the Northeast, the rise in hospitalizations has been minimal, and deaths are nonetheless dropping. In response to the company’s most up-to-date standards, greater than 98 % of People dwell in communities with a low or medium degree of danger.

Even amongst those that are hospitalized, “we’re seeing much less oxygen use, much less I.C.U. stays and we haven’t, luckily, seen any enhance in deaths related to them,” stated the C.D.C.’s director, Dr. Rochelle Walensky. “We’re hopeful that constructive traits will proceed.”

The nation has recorded a few five-fold drop in P.C.R. testing for the virus because the Omicron peak, and so monitoring new instances has develop into tough. However the reported rely is way much less, about 70-fold decrease, stated Dr. Walensky, reflecting “a real and dependable drop in our general instances.”

New subvariants of Omicron, known as BA.2 and BA.2.12.1, have supplanted the earlier iteration, BA.1, which started circulating within the nation in late November and despatched instances hovering to document highs in a matter of weeks.

“After all, much more have been contaminated now, as a result of BA.2 could have contaminated some who averted it to date,” Dr. Hanage stated.

By February, three of 4 youngsters and adolescents within the nation had already been contaminated with the virus, in contrast with one-third of older adults, in keeping with the brand new research.

That so many youngsters are carrying antibodies could provide consolation to folks of these aged 5 and beneath, who don’t qualify for vaccination, since many could have acquired not less than some immunity via an infection.

However Dr. Clarke urged mother and father to immunize youngsters who qualify as quickly as regulators approve a vaccine for them, no matter their prior an infection. Amongst youngsters who’re hospitalized with the virus, as much as 30 % may have intensive care, she famous.

Though a lot of these youngsters additionally produce other medical circumstances, about 70 % of instances of multisystem inflammatory illness, a uncommon consequence of Covid-19 an infection, happen in in any other case wholesome youngsters.

“As a pediatrician and a father or mother, I might completely endorse the kids get vaccinated, even when they’ve been contaminated,” Dr. Clarke stated.

Some consultants stated they had been involved about long-term penalties, even in youngsters who’ve delicate signs.

“Given the very excessive proportion of an infection in children and adults that occurred earlier this yr, I fear concerning the rise in lengthy Covid instances consequently,” stated Akiko Iwasaki, an immunologist at Yale College who’s finding out the situation.

To measure the proportion of the inhabitants contaminated with the virus, the research relied on the presence of antibodies produced in response to an an infection.

C.D.C. researchers started assessing antibody ranges in individuals at 10 websites early within the pandemic, and have since expanded that effort to all 50 states, the District of Columbia and Puerto Rico. The investigators used a take a look at delicate sufficient to establish beforehand contaminated individuals for not less than one to 2 years after publicity.

The researchers analyzed blood samples collected from September 2021 to February 2022 for antibodies to the virus, after which parsed the info by age, intercourse and geographical location. The investigators appeared particularly for a kind of antibody produced after an infection however not after vaccination.

Between September and December 2021, the prevalence of antibodies within the samples steadily elevated by one to 2 proportion factors each 4 weeks. However it jumped sharply after December, growing by almost 25 factors by February 2022.

The proportion of samples with antibodies rose from about 45 % amongst youngsters aged 11 years and youthful, and amongst adolescents aged 12 to 17 years, to about 75 % in each age teams.

By February 2022, roughly 64 % of adults aged 18 to 49 years, about 50 % of these aged 50 to 64 years and about 33 % of older adults had been contaminated, in keeping with the research.

Regardless of the document excessive instances throughout the Omicron surge, the reported statistics could not have captured all infections, as a result of some individuals have few to no signs, could not have opted for testing or could have examined themselves at residence.

In response to one upcoming C.D.C. research, there could also be greater than three infections for every reported case, Dr. Clarke stated.

Noah Weiland contributed reporting from Washington.