(NEXSTAR) — At least three major airlines said they have canceled dozens of flights because illnesses largely tied to the omicron variant of COVID-19 have taken a toll on flight crew numbers during the busy holiday travel season.

Delta Air Lines and United Airlines said they had to cancel dozens of Christmas Eve flights because of staff shortages tied to omicron. United canceled 170 flights, and Delta called off 133, according to FlightAware.

“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 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.”

The airline said it was working to rebook as many people as possible.

Delta said it canceled flights Friday because of the impact of omicron and possibility of bad weather after it had “exhausted all options and resources — including rerouting and substitutions of aircraft and crews to cover scheduled flying.”

It said in a statement that it was trying to get passengers to their destinations quickly.

Germany-based Lufthansa said Friday that it was canceling a dozen long-haul transatlantic flights over the Christmas holiday period because of a “massive rise” in sick leave among pilots. The cancellations on flights to Houston, Boston and Washington come despite a “large buffer” of additional staff for the period.

The airline says it couldn’t speculate on whether COVID-19 infections or quarantines were responsible because it was not informed about the sort of illness. Passengers were booked on other flights.

Lufthansa said in a statement that “we planned a very large buffer for the vacation period. But this was not sufficient due to the high rate of people calling in sick.”

Omicron recently became the dominant COVID variant in the U.S. As of Thursday, the U.S. has surpassed its summer peak of COVID infections with a seven-day average of more than 168,000 cases, according to data from The New York Times.

Last week, the president of the nation’s largest flight attendant union and executives at United Airlines, American Airlines, Southwest Airlines, and Delta Air Lines testified during a Senate Commerce Committee hearing on airline oversight. During the hearing, Southwest Airlines CEO Gary Kelly told the Senate panel that the air in passenger jets’ cabins is so clean that face masks “don’t add much” additional protection against the spread of COVID-19 on planes, The Hill reports. Kelly tested positive for COVID-19 days after the hearing.