The C.D.C. shortened isolation periods as Omicron cases soared.

The Centers for Disease Control and Prevention on Monday shortened by half the isolation period for people who contract the coronavirus, saying that those without symptoms could safely resume mixing with others just five days after their positive test results.

That replaced previous guidance from the agency that infected patients isolate for 10 days.

The new guidance was announced as the highly transmissible Omicron variant is sending daily caseloads soaring, worsening a labor shortage and forcing airlines to cancel thousands of flights.

“The Omicron variant is spreading quickly and has the potential to impact all facets of our society,” Dr. Rochelle Walensky, director of the C.D.C., said. The new recommendations “balance what we know about the spread of the virus and the protection provided by vaccination and booster doses. These updates ensure people can safely continue their daily lives.”

Some places are reporting their worst caseloads of pandemic. Delaware, Hawaii, Massachusetts, New Jersey, New York and Puerto Rico have reported more coronavirus cases in the past week than in any other seven-day period, data show.

The numbers point to the ease with which Omicron is spreading across the United States, even as some studies from overseas suggest that the variant might cause less severe illness. Experts warn that the surge of infections, combined with the fact that tens of millions of Americans remain unvaccinated, could still create a severe strain on the U.S. health system and lead to many more deaths.

On Friday, before holiday interruptions to data reporting began to affect daily case totals, the seven-day national average of new daily cases surpassed 197,000, a 65 percent jump over the past 14 days. Deaths also increased by 3 percent during that time, to a seven-day average of 1,345, according to a New York Times database.

The national record for average daily cases is 251,232, set in January during a post-holiday surge.

Hospitalizations are up, too, although not as much as cases. Nearly 72,000 Americans are hospitalized with Covid-19, an 8 percent increase over two weeks but still just over half of peak levels.

From Dec. 5, there has been a fourfold increase of Covid hospital admissions among children in New York City, where the new variant is spreading rapidly, the New York State Department of Health said in an advisory. About half were under 5, and not eligible for vaccination.

Elective surgeries were put on pause at many hospitals after New York’s governor, Kathy Hochul, declared a state of emergency this month.

In Massachusetts last week, Gov. Charlie Baker said he would activate up to 500 members of the National Guard to help in overburdened hospitals. Many other states have done the same.

Government data show that vaccination is still a strong protector against severe illness. Unvaccinated people are five times as likely to test positive and 14 times as likely to die of Covid compared with vaccinated patients, according to the Centers for Disease Control and Prevention.

Still, only 62 percent of Americans are fully vaccinated, and the nation’s medical infrastructure is dangerously frayed two years into the pandemic as hospitals contend with staff shortages fueled by burnout and early retirements. Speaking on ABC’s “This Week” on Sunday, Dr. Anthony S. Fauci, the nation’s top infectious-disease expert, said: “When you have such a high volume of new infections, it might override a real diminution in severity.”

Data out of South Africa and some European countries suggest that Omicron infections have been milder and are producing fewer hospitalizations. But experts warn that might not be true everywhere, adding that the surge in cases may still flood hospitals in many countries.

“Each place has its own demographics and health care system access and, you know, vaccine distribution,” Akiko Iwasaki, an immunologist and researcher at the Yale School of Medicine, said in an interview.

She added that people in England, Scotland and South Africa could have acquired enough immunity from other infections to be able to deal with this variant, or that there could be intrinsic differences in the pathogenicity of Omicron that results in fewer people needing to be hospitalized.

“We cannot assume the same things will happen to the U.S.,” Dr. Iwasaki said. “That is not a reason to relax our measures here, and we still need to vaccinate those pockets of people who are unvaccinated.”