Thousands of frustrated travelers hoping to fly to Christmas celebrations faced a wave of last-minute cancelations, as a spike in coronaviruses cases resulted in airline workers who had contracted the virus or had been exposed to the fast-spreading Omicron variant.
FlightAware said that more than 1,000 flights in the United States had been canceled, disrupting a holiday season that travelers had hoped would represent a return to normal.
The passengers were stuck in crowded airports from Atlanta to Minneapolis to Washington, D.C., and had to use planes, trains and autos to get to their destinations. In many cases, that was impossible.
Thursday was one of the busiest travel days of the year, with fewer than 300 cancelations in the US, but in the evening carriers began to announce problems.
Kate Modolo, a spokeswoman for Delta Air Lines, said that the carrier was exhausting all of its options and resources, including rerouting and substituting planes and crews to cover scheduled flights.
She said Delta expected to cancel at least 150 more flights over the weekend due to a combination of issues.
A spokeswoman for the Chicago-based carrier said that United Airlines canceled about 185 flights on Friday. Crew members were calling in sick.
There were 120 canceled on Saturday. When possible, the carrier swaps in larger planes to carry more passengers on flights.
The disruptions caused by the virus were too much for stopgap measures to match.
When Omicron starts racing through your pilot corps, you can only be prepared. If your pilot has a cold, they can still fly. A pilot has to stay away for 10 days if they get Covid. That can cause an operation to be snarled.
Airlines for America asked the CDC to shorten the recommended isolation period for employees who test positive for Covid-19 from 10 days to no more than five days.
Sara Nelson, the president of the Association of Flight Attendants union, wrote a letter to the C.D.C. saying that changing protocols should be made by public health professionals, not airlines.
Some carriers based in Australia and India have been hit by a large number of cancellation. In China, more than 1,500 cancellation have taken place over the past few days, which is not uncommon before the emergence of Omicron.
The critical date when airlines expect large numbers of travelers to return home is not known. A lot of decisions are being made so that we don't have to cancel a flight.
As more Omicron community spread, there is a chance of more flight cancellation and delays, according to a spokesman for JetBlue.
It is a strange time in the world. While the Omicron variant now accounts for more than 70 percent of new coronaviruses cases in the United States, President Biden and European leaders are reluctant to impose the kind of unpopular restrictions that were put in place to blunt the first wave of the epidemic.
There is a growing belief that such measures are not worth the trouble. The Omicron variant of the disease is not as severe in South Africa, so they are dropping the restrictions on all but the sick.
The new policy allows people who have tested positive but have no symptoms to gather with others if they wear a mask.
The airlines follow strict policies to keep crew members from becoming carriers of the disease.
The problem is going to be a problem through the holidays.
The lowest cancellation levels since 2014) have been carried by a former commercial pilot who is a spokeswoman for FlightAware.
The focus has been on airline travel because people have been so tired. People have a short memory, and they forget how crowded airline travel was before the Pandemic.
Tramelle Howard, a director at an education nonprofit, said his Delta flight from Nashville to Baton Rouge was canceled on Thursday evening.
Mr. Howard will most likely have to miss seeing his family on Christmas because his flight had been rearranged for Sunday, but he said he was trying to make the best of the situation and was going to relax and watch tv.
Mr. Howard said he was accepting it. It is one of the risks that you take when you travel during the holidays.
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A shortage of airline workers has been created by a wave of coronaviruses.
International travel is still below normal levels, but domestic travel has been better than it was in the early months of 2020.
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There are new treatments. The first two pill treatments for Covid-19 were authorized by the FDA. Some Covid patients who are at higher risk of becoming severely ill will be able to take the new drugs at home.
The first trial of new travel took place over Thanksgiving, with few cancelations and nearly the same number of passengers as last year. They had more room to manage disruptions before the Pandemic. Southwest, American and Spirit airlines all had meltdowns in late summer and fall.
You have this choppiness in trying to get back to normal.
The industry's reserve of on-call pilots is thinner than it used to be, after the Pandemic. A steady stream of news about unruly customers and their encounters with airline staff has not made it easier to recruit.
It is harder to juggle staff if there is a shortage because pilots only fly one model of plane.
Travelers around the country experienced setbacks. People were forced to sleep the night before at the Twin Cities airport, so mats, blankets and pillows lined the floors on Christmas Eve.
Catherine Lynn, a sales specialist in Watersound, Fla., spent more than two hours trying to rebook a flight for her son, who found out his flight had been canceled when he tried to check in on the Delta app.
Ms. Lynn will have to drive to a different airport to pick up her son on Christmas Eve because she was able to book a new flight for Friday evening.
At the end of a line for rebooking flights at Kennedy International Airport in Queens, Cesar Zerrato of Elizabeth, N.J. moaned loudly and drew the attention of a few travelers ahead of him.
Mr. Zerrato was supposed to be in Bogot with his family for Christmas. He was caught in an airport loop. He had traveled back to the Northeast for a flight to Bogot from J.F.K. but that flight was canceled as well.
He was trying to rebook a flight.
The journey was all the time, lines, and Mr. Zerrato described it as being all the time.
He was in New York without his luggage. He was worried his bags would help him get on a plane.
Madeleine Ngo, Sean Piccoli, and Nicholas Bogel-Burroughs contributed to the report.