United Airlines has around 3,000 workers out due to COVID-19 the company’s CEO reported on Tuesday, leading to numerous flight cancellations nationwide.

United Airlines CEO Scott Kirby shared the information in a public letter to team members posted on the company’s website Tuesday as nearly 20,000 flights across multiple airlines in the U.S. have occurred since Christmas Eve.

“Our frontline teams continue to put in a tremendous effort during what I know is an incredibly challenging and stressful time – the Omicron surge has put a strain on our operation, resulting in customer disruptions during a busy holiday season. I appreciate your professionalism, creativity, and caring approach – it’s really made all the difference in helping us manage the impact on our customers,” Kirby wrote.