At least three major airlines have reported cancelling dozens of flights as illnesses largely tied to the Omicron variant of the coronavirus take a toll on flight crew numbers during the busy holiday travel season.

At least 5,000 cancellations are expected during December 23-25, according to Flight Aware, a website that tracks flight data.

Germany-based Lufthansa said on Friday that it was cancelling 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 could not 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: “… 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.”

Worldwide, about 50 million people are projected to be travelling on international flights during the end-of-year holiday season, according to the International Civil Aviation Organization (ICAO).

In the United States, Delta Air Lines and United Airlines said they had to cancel dozens of Christmas Eve flights because of staff shortages tied to Omicron.

“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 to several news outlets. “As a result, we’ve unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport.”