United Airlines says 3,000 employees have tested positive for the coronavirus, necessitating a temporary reduction of the carrier’s schedule. In a memo to employees Tuesday, CEO Scott Kirby said the past few weeks have been a “challenging and stressful time.”

“The Omicron surge has put a strain on our operation, resulting in customer disruptions during a busy holiday season,” Kirby wrote.

He said that on a single day recently nearly one-third of employees called out sick and he noted the impact of the virus on United’s Newark hub.

The New York region was among the first spots in the United States to see a sharp uptick in coronavirus cases, largely driven by omicron.

In reducing its schecule, United follows other carriers, including JetBlue and Alaska Airlines, which in recent days also have announced schedule reductions.

Southwest Airlines however, said Tuesday it has no plans to cut its schedule in January or beyond. Southwest had few flight disruptions leading up to and immediately after Christmas but was forced to cancel several hundred flights around New Year’s Day after winter weather affected its hubs in Chicago and Baltimore.

United has been criticized for requiring employees to be fully vaccinated and firing workers who did not comply with its policy. In his letter, Kirby defended the mandate, saying it spared employees from the worst effects of the coronavirus.

He said none of the 3,000 employees who have tested positive and are vaccinated are hospitalized. Since the policy went into effect, the hospitalization rate among fully vaccinated employees has been a hundredth of that of the general population of the United States, Kirby said. In addition, United has gone eight weeks without a coronavirus-related death among vaccinated employees.

He said that while he knows that, “some people still disagree with our policy, United is proving that requiring the vaccine is the right thing to do because it saves lives.”

United had 67,000 US-based employees as of September, although it wasn’t immediately clear Wednesday whether the 3,000 coronavirus cases noted in Kirby’s letter referenced only US-based workers.

United on Dec. 31 offered triple pay for pilots to pick up trips through most of January. The pilots’ union said sick calls among pilots were at a record.