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Airlines canceled thousands of flights worldwide on Christmas Eve and Christmas Day amid a surge in COVID-19 cases caused by the spread of the recently identified omicron variant.

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The cancellations include more than 510 set to depart on Christmas Eve and over 235 set for Christmas Day within, into or out of the U.S., according to the flight tracking site FlightAware. As of Friday morning, airlines across the globe canceled more than 2,100 flights set to depart on Christmas Eve and more than 1,200 set for Christmas Day

Officials with Germany-based Lufthansa told The Associated Press on Friday that they were canceling a dozen transatlantic flights over the holiday in response to a “massive rise” in sick leave among pilots.

>> Related: Coronavirus: United, Delta, JetBlue and others cancel Christmas Eve, Christmas Day flights

“We planned a very large buffer for the vacation period,” the airline said in a statement obtained by the AP. “But this was not sufficient due to the high rate of people calling in sick.”

In a statement obtained by The New York Times, officials with Delta Airlines said they canceled more than 130 flights on Friday due to “a combination of issues, including but not limited to, potential inclement weather in some areas and the impact of the omicron variant.” Similarly, United Airlines said in a statement to the Times that the viral variant’s “direct impact on our flight crews and the people who run our operation” had prompted them to cancel more than 150 flights Friday.

>> Related: Omicron now dominant US coronavirus strain, CDC says

The cancellations come even as officials with the Transportation Security Administration screen millions of passengers through travel checkpoints nationwide. On Thursday, officials said they screened nearly 2.2 million passengers, nearly the same amount screened on the same day in 2019, before the coronavirus pandemic prevented many from taking to the air.

Reports of COVID-19 infections have been rising in recent days as the U.S. grapples with the arrival of the omicron variant. Much remains unknown about the viral strain, which was first detected in the country on Dec. 1. Officials with the Centers for Disease Control and Prevention said the variant, which has since become the dominant strain across the U.S., has been reported in 110 countries.

>> Related: Omicron projected to cause more COVID-19 infections, fewer hospitalizations, analysts say

Since the start of the pandemic, officials have reported 51.8 million cases of COVID-19 nationwide, resulting in more than 815,000 deaths, according to a tally from Johns Hopkins University. Globally, nearly 278.3 million cases have been reported, resulting in 5.3 million deaths, according to the university.