Travel issues that forced thousands of flight cancellations over the Christmas weekend persisted Monday, a day after a report showed similar issues did not affect holiday sales. The travel issues have been attributed to COVID-19, particularly the Omicron variant, as well as bad weather.

According to flight tracking website FlightAware, as of 4:00 p.m. EST Monday, 1,153 flights “within, into, or out of the United States” had been cancelled. Airlines have canceled roughly 4,000 flights to, from or inside the US since Friday. The video above shows the scenes at various U.S. airports over the past few days.

“One of my best friends is stuck in Mexico right now, and her family was tested positive,” airline passenger Lucy Burke said Sunday. “They got kicked out of their hotel room. And it’s just a whole disaster.”

A major factor in the disruption of holiday travel has been airline staffers calling out sick because of COVID-19. Delta, United, JetBlue and American have all said coronavirus was causing staffing problems. United said it canceled 115 flights Monday due to crews out with COVID-19.

“United and Delta have been vocal in admitting that many of these cancellations are because of COVID, even though they have vaccinated employees, and even though they and other airlines have taken steps to reduce the chance of COVID affecting their workforce,” Atmosphere Research travel industry analyst Henry Harteveldt said. “If you see a lot of people getting sick, you can assume that some of those folks work at the airline you’re supposed to fly this week.”

Winter weather has also affected holiday travel. Sunday snow showers blew into the Pacific Northwest from the Gulf of Alaska, dumping up to 6 inches across the Seattle area. Alaska Airlines said the snowfall led to nearly 250 flight cancellations to or from Seattle on Sunday, with more than 100 more expected Monday.

While holiday travel took a hit, holiday sales increased sharply. Mastercard SpendingPulse, which tracks all kinds of payments including cash and debit cards, reported holiday sales had risen 8.5% from a year earlier.

“Shoppers were eager to secure their gifts ahead of the retail rush, with conversations surrounding supply chain and labor supply issues sending consumers online and to stores in droves,” Mastercard senior advisor Steve Sadove said in a news release. “Consumers splurged throughout the season, with apparel and department stores experiencing strong growth as shoppers sought to put their best dressed foot forward.”

The 8.5% number is on the low end of what the National Retail Federation had predicted for “holiday sales during November and December.”