United, Delta and other U.S. airlines have canceled more than 600 Christmas Eve flights as a surge in COVID-19 cases impacts their staff, the airlines said. The cancellations came as the Transportation Safety Administration said the number of people traveling for the holiday is returning to pre-pandemic levels.

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

As of late Friday afternoon, 627 flights within, to and from the U.S previously scheduled for December 24 have been canceled, according to FlightAware. United had canceled over 175 flights, Delta called off 150, and JetBlue scrubbed 72, data from the real-time tracking service showed.

“Delta teams have exhausted all options and resources — including rerouting and substitutions of aircraft and crews to cover scheduled flying,” the airline said in a statement Thursday.  

Airlines apologized for the disruptions and said they were trying to rebook passengers.

Around the world, a total of 2,295 flights were canceled Friday, adding to the previous day’s 2,232 global cancellations. 

The Associated Press reports that Germany-based Lufthansa said Friday it was canceling 12 transatlantic flights over the Christmas holiday period because of a “massive rise” in sickouts by pilots. The flights were to head to Houston, Boston and Washington.

Lufthansa said it had arranged for a “large buffer” of additional staff for the period but still needed to cancel the flights. The airline wouldn’t speculate on whether COVID-19 infections or quarantines were behind the sickouts because it wasn’t told which illnesses were involved. Passengers were booked on other flights. 

Delta CEO Ed Bastian was among those who have called on the Biden administration to take similar steps or risk further disruptions in air travel. On Thursday, the U.S. shortened COVID-19 isolation rules for health care workers only.

American and Southwest Airlines told CBS DFW Thursday they have no plans to cancel any flights due to COVID-19 staffing issues.

Millions flying despite Omicron

Still, holiday travel this year has been brisk. FlightAware says it has tracked more than 120,000 arrivals in the past 24 hours.

“We’re 27% ahead of where we were last year,” said Robert Sinclair, a senior manager of public affairs at AAA Northeast. “Without a doubt, people have more confidence with the vaccines and the boosters.” 

The cancellations come as millions of people are expected to travel for the holidays, despite the nationwide surge in COVID-19 cases led by the Omicron variant. While some travelers canceled holiday plans because of rising case numbers, many others kept to their vacations during some of the year’s busiest travel days. 

The Transportation Security Administration said it expects to screen nearly 30 million people from December 20 through January 3, compared with nearly 44 million during the last holiday season before the pandemic. Six million people are expected to fly, thanks in part to vaccinations.

The TSA said its agents screened more than 2 million people on Wednesday, exceeding the number of travelers on that day in 2019.

The Centers for Disease Control and Prevention urges those planning to travel to be fully vaccinated against COVID-19. People are also advised to avoid crowded and poorly ventilated spaces and to wear a well-fitting face mask when in public indoor settings. 

The CDC has also warned of spiking COVID-19 cases as the fast-spreading Omicron variant infects many throughout the nation. Omicron has become the dominant strain of the virus in the U.S. and health officials are urging people to get fully vaccinated and boosted.