Christmastime already has a reputation as the busiest, most stressful time to fly, but this year a wave of COVID-19 induced cancellations and delays has upended travel plans for thousands.

On Monday, there were 4,074 delays nationwide with 1,119 cancellations, while worldwide there were 10,405 delays and 2,637 cancellations, according to FlightAware.

As of Monday afternoon, Salt Lake City International Airport had 43 cancellations and 178 delays. Over 50,000 people are expected to have come through the airport doors over two days — 26,000 on Sunday and over 27,000 forecasted for Monday, according to a spokesperson.

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Nationwide, more than 1.53 million people passed through airport security Saturday, according to CNN.

It’s a combination of snowy weather in parts of the U.S. and staffing problems among airlines due to a nationwide increase COVID-19 cases, driven by the highly transmissible omicron variant.

Much of the West received snow over the holidays, while the country as a whole saw 214,449 new cases of COVID-19 on Sunday, an 83% increase over 14 days, according to the New York Times.

That’s put a sizable speed bump in the holiday plans of many.

“I just happened to look at the app on the way to the airport, just to look at the boarding pass and it said ‘oh yeah by the way your flight’s canceled, we moved you to an hour earlier flight,’ which I would've missed regardless,” Samantha Stolworthy told KSL-TV.

Stolworthy said the process of finding a new flight was convoluted — “You can’t call them because it’s a four-hour hold time, so you’re kind of just in limbo.” So she resorted to driving nearly 10 hours to California.

In a statement, United Airlines said the rising COVID-19 case counts “had a direct impact on our flight crews and the people who run our operation. As a result, we’ve unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport.”

And prior to Christmas, Delta Air Lines said in a statement that the company has “exhausted all options and resources — including rerouting and substitutions of aircraft and crews to cover scheduled flying — before canceling around 90 flights.”

Alaska Airlines, which canceled over 120 flights Monday and warned on its blog that more should be expected, chalked most of it up to the weather. Most of the cancellations have been confined to the Pacific Northwest.

“Snow and wintry conditions are creating a bit of a bah-humbug for our operations to and from Seattle,” the company said.

Salt Lake City International Airport spokeswoman Nancy Volmer reiterated the tips airports have long emphasized — check with your airline prior to leaving for the airport to make sure your flight hasn’t been delayed or canceled, leave two hours prior to boarding for domestic flights and three hours for international trips, and if possible check in online before arriving at the airport.

She also stressed that passengers practice good hygiene, and that masks are required on all flights.

On Monday, Dr. Anthony Fauci, the White House medical adviser on the coronavirus, said that a vaccination requirement for domestic air travel could help with some of the problems facing airports right now, calling it “reasonable to consider.”

“When you make vaccination a requirement, that’s another incentive to get more people vaccinated,” Fauci said on MSNBC. “If you want to do that with domestic flights, I think that’s something that seriously should be considered.”