The Centers for Disease Control and Prevention on Tuesday significantly revised downward its estimate of the percentage of new COVID-19 infections in the U.S. caused by the omicron variant, raising questions about the agency’s understanding of the new strain.

The CDC said omicron accounted for 22.5% of all COVID-19 cases for the week ending Dec. 18, a stunning drop from its initial estimate of 73% of all cases.

The agency said omicron accounted for about 59% of all U.S. infections as of Saturday, meaning that the delta variant still accounted for about 41% of infections.

The earlier estimate from the CDC’s Nowcast model, which is based on genomic-sequencing data, caused alarm by indicating that omicron had spiked from 3% of all cases in the U.S. to 73% nearly overnight.

A CDC spokeswoman attributed the major disparity to “a wide predictive interval posted in last week’s chart, in part because of the speed at which omicron was increasing.”

“We had more data come in from that timeframe and there was a reduced proportion of omicron,” the spokeswoman said in a statement. “It’s important to note that we’re still seeing steady increase in the proportion of omicron.”

Former Food and Drug Administration Commissioner Dr. Scott Gottleib said on Twitter: “Setting aside the question of how the initial estimate was so inaccurate, if CDC’s new estimate of Omicron prevalence is precise then it suggests that a good portion of the current hospitalizations we’re seeing from Covid may still be driven by Delta infections.”

The new estimate prompted more criticism of the CDC. New York state Assemblyman Tom Abinanti, a Democrat from Greenburgh, said on Twitter that the federal government “prematurely cut supply of now desperately needed but unavailable monoclonal antibody treatments for delta patients when CDC wrongly estimated omicron was 73% not 23%.”

Health officials were reportedly stockpiling the monoclonal antibody treatment sotrovimab after Thanksgiving in preparation for the variant to become dominant in the U.S.

The major revision on omicron’s spread comes a day after U.S. health officials cut isolation restrictions for Americans who catch the coronavirus from 10 to five days, and they similarly shortened the time that close contacts need to quarantine. The CDC said there’s growing evidence that people with the coronavirus are most infectious in the two days before and three days after symptoms develop.

Early research suggests omicron may cause milder illnesses than other versions of the coronavirus. But the sheer number of people becoming infected — and therefore having to isolate or quarantine — threatens to crush the ability of hospitals, airlines and other businesses to stay open, according to experts.

Omicron is highly contagious, and pediatric hospitalizations for COVID-19 are rising nationwide. But New York City has far outpaced the nation, with a 400% increase in pediatric hospitalizations for the virus in December.

New York City will ramp up testing of public school students for COVID-19 next week and will stop its policy of quarantining whole classrooms of exposed students in an effort to keep schools open, officials said Tuesday, even as pediatric hospitalizations are rising.

New York Gov. Kathy Hochul, a Democrat, called remote learning “a failed experiment” as she announced the state will provide city schools with 2 million at-home test kits. Mayor Bill de Blasio and Mayor-elect Eric Adams said the tests can be sent home with students if a classmate tests positive, and students who test negative won’t need to quarantine.

“Your children are safer in school, the numbers speak for themselves,” Mr. Adams said.

Meanwhile, President Biden on Tuesday revoked travel restrictions that his administration imposed on eight countries in southern Africa where the omicron variant was first detected.

He said U.S. health officials “have made substantial progress in understanding” the fast-spreading variant.

“Importantly, scientific experts have determined that people who are vaccinated against COVID-19 are protected against severe disease and hospitalization from the omicron variant,” Mr. Biden said. “Moreover, the omicron variant has now spread to more than 100 countries, and it is prevalent in the United States.”

Despite the early indications of milder symptoms with omicron, the spread of the new variant is having a broad impact across the U.S. over the holidays.

The seven-day average of new COVID-19 cases in Philadelphia reached a high this week of 1,407 new cases per day, eclipsing the previous record of 1,235 set on Dec. 8, 2020, the Philadelphia Inquirer reported.

Apple closed all of its roughly one dozen New York City locations to browsing, saying the stores will be limited to picking up online orders and walk-in services such as the Genius Bar.

“We regularly monitor conditions and we will adjust both our health measures and store services to support the wellbeing of customers and employees,” the company said in a statement.

In Atlanta, Emory University’s president said the school is switching to virtual classes to start the spring semester because of a national surge in COVID-19 cases fueled by the omicron variant.

The judge presiding over the sex trafficking trial of Ghislaine Maxwell cited an “astronomical spike” in the number of coronavirus cases in New York City as she explained Tuesday why she was urging jurors to work longer hours.

Judge Alison J. Nathan said aloud what had largely gone unmentioned in her previous requests to get the jury to work an extra day last week and longer hours this week as it decides whether Ms. Maxwell recruited and groomed teenage girls to be sexually abused by Jeffrey Epstein. The jury declined to work an extra day last week.

Maryland’s judiciary announced the postponement of jury trials and reduced other court operations. Jury trials scheduled between Wednesday and Feb. 8 will be rescheduled.

On Monday, state health officials reported 5,376 new cases, an increase in hospitalizations to 1,714 and an increase in the seven-day testing positivity rate to more than 16.5%.

Longtime U.S. Rep. Bobby Rush, Illinois Democrat, said late Monday that he tested positive for a breakthrough case of COVID-19. The 75-year-old cancer survivor said he was “feeling fine” and had no symptoms.

The U.S. late last month had restricted travel from eight African countries — Botswana, Eswatini, Lesotho, Malawi, Mozambique, Namibia, South Africa and Zimbabwe — after scientists in southern Africa first discovered the omicron variant.

Biden administration officials defended the move, arguing it allowed more time to prepare for the spread of the omicron variant before it reached the U.S.

• This article is based in part on wire-service reports.

For more information, visit The Washington Times COVID-19 resource page.

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WASHINGTON — U.S. health officials’ decision to shorten the recommended COVID-19 isolation and quarantine period from 10 days to five is drawing criticism from some medical experts and could create confusion among many Americans.

The move has raised questions about how the guidance was crafted and why it was changed now, in the middle of another wintertime spike in cases, this one driven largely by the highly contagious omicron variant.

Monday’s action by the Centers for Disease Control and Prevention cut in half the recommended isolation time for Americans who are infected with the coronavirus but have no symptoms. The CDC similarly shortened the amount of time people who have come into close contact with an infected person need to quarantine.

The new guidance was issued amid warnings from the business community that the spike in cases could soon cause widespread staffing shortages because of workers being forced to stay home. Already, thousands of airline flights have been canceled over the past few days in a mess blamed on omicron.

CDC officials said the guidance is in keeping with growing evidence that people with the virus are most infectious in the first few days.

But other medical experts questioned why the agency’s guidelines allow people to leave isolation without testing.

“It’s frankly, reckless to proceed like this,” said Dr. Eric Topol, founder and director of the Scripps Research Translational Institute. “Using a rapid test or some type of test to validate that the person isn’t infectious is vital.”

“There’s no evidence, no data to support this,” he added.

Just last week, the CDC loosened rules that previously called on health care workers to stay off the job for 10 days if they test positive. The new recommendations said workers could go back after seven days if they test negative and don’t have symptoms.

Early research suggests omicron may cause milder illnesses than earlier versions. But the sheer number of people becoming infected – and therefore having to isolate or quarantine – threatens to crush the ability of hospitals, airlines and other businesses to stay open, experts say.

“Not all of those cases are going to be severe. In fact, many are going to be asymptomatic,” CDC Director Dr. Rochelle Walensky said Monday. “We want to make sure there is a mechanism by which we can safely continue to keep society functioning while following the science.”

Louis Mansky, director of the Institute for Molecular Virology at the University of Minnesota, said there is a scientific basis to the CDC’s recommendations.

“When somebody gets infected, when are they most likely to transmit the virus to another person?” he said. “It’s usually in the earlier course of the illness, which is typically a day or two before they actually develop symptoms and then a couple of days to three days after that.”

Research, including a study published in the journal JAMA Internal Medicine in August, backs that up, though medical experts cautioned that nearly all of the data predates omicron.

The CDC has been under pressure from the public and the private sector, including Delta Air Lines, to explore ways to shorten the isolation time. Airline officials earlier this month sent a letter to the CDC proposing five days of isolation for fully vaccinated people who become infected, with a “testing protocol” to leave isolation.

Mansky said CDC probably didn’t include exit testing in its guidelines for logistical reasons: There is a run on COVID-19 rapid tests amid the spike in cases and the busy holiday travel season. In many places, at-home tests are difficult or impossible to find.

The CDC is “driven by the science, but they also have to be cognizant of the fact of, you know, what are they going to tell the public that they’ll do,” Mansky said. “That would undermine CDC if they had guidance that everybody was ignoring.”

Marshall Hatch, senior pastor of New Mount Pilgrim Church on Chicago’s West Side, said he is bracing for some confusion in his congregation. The church has been a strong advocate for testing, vaccinations and booster shots.

Hatch said the CDC’s latest guidance is confusing and “a little incongruous.”

“Either we’re in a surge that we need to take very seriously or are we winding down the pandemic and that’s why we’re shortening the isolation and quarantine times,” he said Tuesday. “They might want to give us a little more information to go with.”

Hatch said some members of the largely Black congregation, particularly senior citizens, are skeptical of information from government.

The CDC move follows global efforts to adjust isolation rules, with policies differing from country to country.

England last week trimmed its self-isolation period for vaccinated people who have tested positive for COVID-19 to seven days in many cases, provided two negative lateral flow tests are taken a day apart.

The French government said Monday that it will soon relax its isolation rules, although by exactly how much isn’t yet clear.

Health Minister Olivier Veran said the rule changes will be aimed at warding off “paralysis” of public and private services. By some estimates, France could be registering more than 250,000 new infections per day by January.

Italy, meanwhile, is considering doing away with a quarantine altogether for those who have had close contact with an infected person as long they have had a booster shot. Projections indicate as many as 2 million Italians could be put in quarantine over the next two weeks as the virus spreads.

The U.S. airline industry applauded the CDC move.

“The decision is the right one based upon science,” said the lobbying group Airlines for America.

But the head of a flight attendants union criticized the change, saying it could lead businesses to pressure sick employees to come back before they are well.

If that happens, “we will make clear it is an unsafe work environment, which will cause a much greater disruption than any ‘staffing shortages,’” warned Sara Nelson, president of the Association of Flight Attendants-CWA International.

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Associated Press writers Laura Ungar in Louisville, Kentucky; Colleen Barry in Milan, Italy; and Tali Arbel and Mike Stobbe in New York contributed.

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