At least three major airlines have reported canceling dozens of flights as illnesses largely tied to the omicron variant of COVID-19 take a toll on flight crew numbers during the busy holiday travel season.

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.”

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