Thousands of flights canceled over weekend as Omicron thins crews.

Airlines canceled thousands of flights on one of the busiest weekends of the year because of the Omicron variant of the coronaviruses. Thousands of travelers were frustrated by the disruptions.

About 2,000 U.S. flights were canceled over the Christmas holiday weekend, with more than 3000 grounded worldwide, according to FlightAware. More than 1,100 flights were canceled in the U.S. on Sunday alone.

The current wave of Covid-19 cases, which has risen in the United States to a level not seen since last winter, contributed significantly to the recent groundings.

An airline spokesman said the airline had seen an increase in sick calls.

The majority of flights on Sunday had been canceled by the afternoon.

Dan Landson, a Southwest spokesman, said that Southwest Airlines canceled 63 flights because of weather. He said in an email that there had been no operational issues related to Covid. Southwest is one of the four largest US carriers.

The situation at airports has gotten worse because of the increase in travelers this year. The numbers on Christmas Eve and Christmas Day were higher than last year, and some of the figures were even higher than two years ago, when Americans were not aware of a virus that was spreading around the world.

The worst of the cancellation may be over in the US. Delta was expecting to cancel about 200 flights on Sunday, less than the 300 it had predicted a day earlier, and it was forecasting only 40 cancellation on Monday.

On a Sunday, airlines expect a lot of travel. The Omicron variant, which is responsible for more than 70 percent of the new coronaviruses in the United States, has helped push daily case averages in the United States above 200,000 for the first time in nearly a year.

An airline trade group wants the CDC to shorten the recommended isolation period for fully vaccine-vaccinated employees who test positive to a maximum of five days.

The C.D.C. would alleviate some of the staffing pressures and set up airlines to help millions of travelers returning from their holidays, said a spokesman for JetBlue.

The flight attendants union believes that public health professionals should make the decisions about the recommended isolation times.

Some of the delays had nothing to do with the Pandemic. Alaska Airlines instituted an extensive program to keep crews healthy and even had members of its management team who are trained to be crew members step in.

On Saturday and Sunday, there were only a few cancellation related to crew exposure to the coronaviruses. It had canceled more than 100 flights because of the cold and snowy weather in the Pacific Northwest, which affected its hub, Seattle-Tacoma International Airport.

The company said in a statement Sunday that the snow and wintry conditions are creating a bit of a bah-humbug for their operations to and from Seattle.