Thousands of flights scheduled for the evening of Christmas Eve were canceled, leaving travelers scrambling to change their holiday plans.

At least three major airlines scrubbed flights, citing COVID-19 infections among flight crews and the spread of the omicron variant. More than 2,000 flights were canceled worldwide, and more than 450 within the United States, according to CNN.

In a statement on Friday, Lufthansa said it canceled a dozen trans-Atlantic flights because of a "massive rise" of sick leave requests among its pilots.

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"We planned a very large buffer for the vacation period," Lufthansa said in a statement. "But this was not sufficient due to the high rate of people calling in sick."

The airline said it could not speculate on whether COVID-19 infections or quarantines were responsible because those affected had not disclosed their illnesses. The airline also said it shifted any affected passengers to other flights.

United and Delta airlines also reported canceling flights.

"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 Airlines said 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."

United said it would rebook as many people as possible.

Delta said in a statement on Friday that it canceled flights due to the omicron variant and concerns about bad weather, noting it had "exhausted all options and resources — including rerouting and substitutions of aircraft and crews to cover scheduled flying." The airline also said it would try to get passengers to their destinations quickly.

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The omicron variant has become pervasive across the U.S. and the rest of the world.

The Centers for Disease Control and Prevention released data on Tuesday showing omicron cases accounted for more than 73% of new reported COVID-19 cases reported the prior week.