Thousands of Americans hoping to get home for the holidays are at risk of being stranded.
On Thursday, United and Delta together canceled more than two hundred Christmas Eve flights due to COVID-19.
Both said they were working to contact passengers so they would not be left waiting at airports.
The rapidly spreading Omicron variant has taken a toll on flight crews and other workers.
In a statement, Delta airlines said it had "exhausted all options and resources before canceling around 90 flights for Friday."
While, United said it had been forced to cancel 120 flights due to staff shortages.
The airlines are lobbying for the quarantine guidelines to be relaxed for those fully vaccinated who experience breakthrough COVID-19 infections.
On Tuesday, Delta Chief Executive Ed Bastian asked the head of the U.S. Centers for Disease Control and Prevention
to cut the isolation period to five days from the current 10, citing the impact on the carrier's workforce.
That request was echoed both by Airlines for America, a trade group representing major cargo and passenger carriers.