Delta Air Lines and United Airlines said they canceled flights because of staff shortages tied to the omicron variant. Delta canceled 145 flights on Friday and 111 for Christmas Day, according to FlightAware. (Other factors, such as weather, are also causing cancellations.) United called off 175 flights on Friday and 69 on Saturday.

Not all airlines said COVID was disrupting their travel schedules. American Airlines said it had "nothing to report," while Southwest Airlines said "things are running smoothly." JetBlue did not respond to a request for comment.Flight delays and cancellations tied to staffing shortages have been a regular problem for the U.S. airline industry this year. Airlines encouraged workers to quit in 2020, when air travel collapsed, and were caught short-staffed this year as travel recovered.