At least three major airlines say 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.

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.

U.S.-based Delta Air Lines and United Airlines say they had to cancel some Christmas Eve flights because of staff shortages tied to Omicron.

FlightAware shows United canceled 170 flights, and Delta called off 133.