What you need to know about the omicron variant 2:06
(CNN Business) -
Airlines canceled thousands of flights over Christmas weekend, including more than 1,000 domestic flights in the United States, as staff and crews fell ill during the surge in omicron cases.
Globally, airlines have canceled about 5,700 flights for Christmas Eve, Christmas and the day after Christmas, according to FlightAware.
That includes about 1,700 flights in, to or out of the United States.
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Operational problems occur at airlines as millions still fly despite the increase in coronavirus cases.
The TSA says it screened 2.19 million people at airports across the country on Thursday, the highest number since the surge in vacation travel began a week ago.
Airlines cancel hundreds of flights before Christmas 3:43
More than a thousand flights canceled in the US
On Thursday, United Airlines said it had to "cancel some flights" due to the omicron variant of the coronavirus.
"The nationwide surge in omicron cases this week has had a direct impact on our flight crews and the people who run our operation," said a United memo obtained by CNN.
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United canceled 201 flights on Friday, representing 10% of its total schedule, and 238 flights on Saturday, representing 12% of its overall schedule, according to flight tracking site FlightAware.
United said it "notifies affected customers before they arrive at the airport," according to a company statement.
"We apologize for the interruption and are working hard to rebook as many people as possible and get them on track for the holidays."
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Later Thursday night, Delta Air Lines also canceled flights.
The airline canceled 173 flights on Christmas Eve, according to FlightAware.
Delta said the cancellations are due to multiple problems, including the omicron variant.
"We apologize to our customers for the delay in their vacation travel plans," Delta said in a statement.
"The people at Delta work hard to get you where you need to be as quickly and safely as possible on the next available flight."
Additionally, JetBlue canceled 80 flights, or about 7% of its overall schedule, the day before Christmas.
Alaska Airlines said in a statement that it canceled 17 flights due to the omicron on Thursday and canceled 11 flights on Friday.
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Thousands of international flights canceled
China Eastern has canceled 474 flights, or 22% of its total operation, according to FlightAware.
Similarly, Air China canceled about 190 flights, or 15% of its overall schedule.
Air India, Shenzhen Airlines, Lion Air and Wings Air also canceled dozens of flights.
Andy Rose, Sharif Paget, Ramishah Maruf, Eric Levenson, and Carma Hassan contributed to this report.