NEW YORK (AP) — Flight cancellations that disrupted holiday travel stretched into Monday, with major U.S. airlines each canceling dozens of flights.

Staffers calling out sick because of COVID-19, particularly since the emergence of the omicron variant, have left airlines short in recent days. According to FlightAware, which tracks flight cancellations, airlines have canceled roughly 4,000 flights to, from or inside the U.S. since Friday.

Delta, United, JetBlue and American have all said that the coronavirus was causing staffing problems, and European and Australian airlines also canceled holiday-season flights because staff were infected, but weather and other factors played a role as well.

Winter weather in the Pacific Northwest led to nearly 250 flight cancellations to or from Seattle on Sunday, said Alaska Airlines, and the airline expects more than 100 flight cancellations Monday. But it says that crew calling out sick because of COVID-19 is no longer a factor.

United said it canceled 115 flights Monday, out of more than 4,000 scheduled, due to crews out with COVID-19.

Flight delays and cancellations tied to staffing shortages have been a consistent problem this year. Airlines encouraged workers to quit in 2020 when air travel collapsed, and were caught short-staffed this year as air travel rebounded faster than almost anyone had expected.

Airlines have called on the Biden administration to shorten the guidelines for the isolation period for vaccinated workers who get COVID-19, in order to ease staffing shortages. The union for flight attendants has pushed back against that, saying the isolation period should remain 10 days.