At least 2,300 more flights were canceled globally today, including about 800 U.S. flights, as travel disruptions from one of the year’s busiest weekends for flying spilled into the workweek.

Over the holiday weekend, airlines canceled thousands of flights as the omicron variant of the coronavirus hit flight crews. In all, about 2,300 U.S. flights were canceled Saturday and Sunday of the Christmas holiday weekend, with more than 3,500 more grounded globally, according to FlightAware, which provides aviation data. On Sunday alone, more than 1,300 U.S. flights and nearly 1,700 additional ones worldwide were canceled.

While some of the groundings were caused by bad weather and maintenance issues, several airlines acknowledged that the current wave of coronavirus cases, which has risen in the United States to levels not seen since last winter, contributed significantly. A JetBlue spokesman said the airline had “seen an increasing number of sick calls from omicron.”

Twelve percent of JetBlue flights, 6% of Delta Air Lines flights, 5% of United Airlines flights and 2% of American Airlines flights Sunday had been canceled, according to FlightAware.

Southwest Airlines canceled 68 flights, or 1%, according to FlightAware, because of weather, said Dan Landson, a Southwest spokesman.

“We haven’t had any operational issues related to COVID,” he said in an email.

The stock prices of United, Delta, American and Southwest — the four largest U.S. carriers — were about 2% to 3% lower today.

Traveling rebounded sharply this year, making the situation at airports worse: Roughly 2 million people passed through screening checkpoints each day last week and Sunday, according to the Transportation Security Administration. The numbers on Christmas Eve and Christmas Day were much higher than last year, and some figures even exceeded those of the same days two years ago, when virtually no Americans were aware of a virus beginning to circulate halfway around the world.

There were hints that the worst of the cancellations may be over in the United States. For instance, Delta had expected to cancel about 200 flights Sunday, fewer than the 300 it had predicted a day earlier, according to a spokeswoman. It is forecasting only 40 cancellations today.

On the other hand, airlines also expect lots of travel Jan. 2, a Sunday. And the omicron variant, which is now responsible for more than 70% of the new U.S. coronavirus cases, has already helped push daily case averages in the United States above 200,000 for the first time in nearly 12 months, according to The New York Times’ coronavirus tracker.

An airline trade group has asked the Centers for Disease Control and Prevention to shorten the recommended isolation period for fully vaccinated employees who test positive to a maximum of five days, from 10 days, before they can return with a negative test.

“Swift and safe adjustments by the CDC would alleviate at least some of the staffing pressures and set up airlines to help millions of travelers returning from their holidays,” said Derek Dombrowski, a JetBlue spokesman.

The flight attendants’ union, however, has argued that reductions in recommended isolation times should be decided on “by public health professionals, not airlines.”

Some of this weekend’s delays had little to do with the pandemic. Alaska Airlines had 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, said a spokeswoman, Alexa Rudin.

On Saturday and Sunday, it had only a handful of cancellations related to crew exposures to the coronavirus, according to Rudin. Yet it had canceled 170 flights those two days, according to FlightAware, including 21% of its Sunday flights, because of unusually cold and snowy weather in the Pacific Northwest, which affected its hub, Seattle-Tacoma International Airport.

This article originally appeared in The New York Times.

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