Airlines canceled hundreds of Christmas Eve flights as Covid hit crews.

More than 2,000 flights were cancelled worldwide, more than 500 of them U.S. flights. United Airlines canceled more than 170 mainline flights Friday, about 9% of its schedule, according to flight-tracking site FlightAware.

“The nationwide spike in Omicron cases this week has had a direct impact on our flight crews and the people who run our operation,” United said in a statement on Thursday. “As a result, we’ve unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport.”

The disruptions come after airline executives said they expect some of the busiest days since the pandemic began over the year-end holidays, despite the surge in Covid infections, driven by the omicron variant. The Transportation Security Administration screened close to 2.2 million people on Thursday, up from 846,500 but shy of the more than 2.5 million who passed through TSA checkpoints two years earlier, before the pandemic.

United said it was trying to rebook as many travelers as possible. It announced most of the cancellations a day before.

U.S. airline executives this week asked the Centers for Disease Control and Prevention officials to loosen quarantine guidelines for fully vaccinated individuals who test positive for Covid to five days from the current 10, citing potential staffing shortages and flight disruptions. The CDC loosened its guidelines for healthcare workers on Thursday.

Delta canceled more than 140 flights, or 7% of its Friday’s schedule, citing bad weather in the Salt Lake City and Seattle area, two of its hubs, and the impact of the fast-spreading omicron variant. Delta cancelled more than 100 Saturday flights while United canceled about 70. JetBlue Airways canceled more than 70 flights on Friday, about 7% of its daily operation.

“Delta teams have exhausted all options and resources — including rerouting and substitutions of aircraft and crews to cover scheduled flying — before canceling around 90 flights for Friday,” the airline said in a statement. “We apologize to our customers for the delay in their holiday travel plans. Delta people are working hard to get them to where they need to be as quickly and as safely as possible on the next available flight.”

More than 90% of Delta employees and more than 96% of United’s U.S. staff are vaccinated, the airlines have said.

Delta and United’s flight disruptions are far fewer than the mass cancellations some airline customers faced this summer and fall as carriers including American and Southwest grappled with staffing shortages and bad weather.

Both of those airlines offered staff extra compensation to work peak holiday trips and meet attendance goals. Southwest had just one cancellation on Friday and American had 11, according to FlightAware.