Over 2,000 flights were on Monday canceled as Covid-19 cases surge across the world.
According to FlightAware, about 900 of the more than 2,400 canceled flights, were within, into or out of the United States.
Over 6,500 flights are delayed.
Airlines canceled more than 6,000 flights globally on Christmas Eve, Christmas and the day after Christmas.
Over 1,200 flights were canceled and more than 5,000 were delayed on Sunday alone in the US as staff and crew call out sick.
The cancellations come at the busiest time of year for air travel.
The US Transportation Security Administration said it screened millions of people each day over the holiday weekend, peaking at 2.19 million travelers on Thursday, December 23.
On Wednesday, more people passed through TSA checkpoints than on the same day in 2019.
Alaska Airlines, which canceled 105 flights of 15% of its operations Monday, attributed its cancellations and delays on winter weather in the Pacific Northwest.
“Snow and wintry conditions are creating a bit of a bah-humbug for our operations to and from Seattle, including flight cancellations and delays,” Alaska Airlines said on its blog Sunday.
Saturday air travel was a bit slower because of the flight cancellations: More than 1.53 million people passed through security checkpoints Saturday.
United Airlines (UAL) said last week it had to cancel hundreds of flights because it lacked enough crew members to safely fly all of its scheduled routes.
“The nationwide spike in Omicron cases this week has had a direct impact on our flight crews and the people who run our operation,” said a United memo obtained by CNN.
Delta (DAL) said it was working to get all stranded travelers home as quickly as possible.
“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.”