Delta, United, and other U.S. airlines canceled more than 500 Christmas Eve flights as a surge in COVID-19 cases impacts their staff.

According to flight tracking site FlightAware, as of midday Friday United has canceled 175 flights for Christmas Eve and 69 on Saturday. Delta dropped 145 flights on Friday and 111 for Christmas Day. Additionally, JetBlue canceled more than 50 flights.

Of the nearly 3,400 flights that were canceled globally on Friday and Saturday, at least half of the cancellations are by Chinese airlines, whereas 20 percent of affected flights are to, from or within the U.S.

“The nationwide spike in omicron cases this week has had a direct impact on our flight crews and the people who run our operation,” United said in a statement. “As a result, we’ve unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport.”

The airline added, “We’re sorry for the disruption and are working hard to rebook as many people as possible and get them on their way for the holidays.”

Meanwhile, Delta told the Associated Press that the “flight cancellations are due to a combination of issues, including but not limited to, potential inclement weather in some areas and the impact of the omicron variant.”

“We apologize to our customers for the delay in their holiday travel plans,” Delta said in a statement. “Delta people are working hard to get them to where they need to be as quickly and as safely as possible on the next available flight.”

Not all airlines have been affected. American Airlines said it had “nothing to report,” while Southwest Airlines said “things are running smoothly.”

The cancellations come as millions of people plan to travel for the holidays. CBS News reports the TSA screened more than two million people on Wednesday, which exceeds the number of people that traveled through airports on December 23, 2019.