Airlines canceled more than 2,100 flights into, out of, or within the United States Sunday, bringing the total number of canceled U.S. flights since Christmas Eve to more than 15,000, USA Today reports.

Staffing shortages continue to plague airlines as the Omicron strain of COVID-19 infects some employees and leaves others afraid to go to work. According to BBC, even the offer of increased pay has been ineffective at luring flight crews back.

On New Year’s Day, almost 4,400 flights were canceled worldwide, The Washington Post reports. More than 2,600 of those were flights for which the U.S. was either the origin, the destination, or both.

Thousands of travelers were (and many remain) stranded.

Chicago, which saw over 1,000 flights into or out of its two major airports scrubbed Saturday, was hit by winter storms in addition to the staffing issues.

Travelers are encouraged to check their flight status before heading to the airport and to be aware that, if the flight on which they’re rebooked is not convenient, they have the right to request a refund instead.