On Monday, more than 950 flights in and out of the U.S. had been canceled and more than 2,400 delayed by noon, according to FlightAware.com, which tracks global flight operations. 

A full 18% of Alaska Airlines flights were canceled on Monday, and another 12% were delayed, according to FlightAware. JetBlue and Spirit were also hit hard, each canceling 6% of their flights and delaying more than 10%.

To put that in perspective, prior to the pandemic, 2.15% of U.S. flights were canceled in 2019, according to the Bureau of Transportation Statistics.

Wave of cancellations

Not all airlines—and therefore not all travelers—have suffered equally.

United Airlines, for instance, canceled 115 flights on Monday due to staffing issues related to Omicron cases—about 2.9% of its more than 4,000 scheduled flights, a spokesperson for the airline tells Fortune. (FlightAware put the number at 4%.)

“The nationwide spike in Omicron cases has had a direct impact on our flight crews and the people who run our operation,” United said in a statement. Throughout the weekend, United said it contacted passengers early if their flight was set to be canceled to give them time to rebook or make other plans. So far, about 50% of United passengers have arrived at their final destination either early or within four hours of their originally scheduled flight, the airline said.

American Airlines has also canceled about 2% of its flights on Monday so far. “Unfortunately a number of COVID-related sick calls led us to make the difficult decision to precancel some flights scheduled for today,” an American Airlines spokesperson said in a statement to Fortune.

Meanwhile a Southwest Airlines spokesperson told Fortune on Monday that the company has not experienced operational issues this holiday period due to COVID outbreaks among its personnel. “Out of our more than 3,600 scheduled flights, we’ve canceled about 50 because of winter weather conditions” this morning, Southwest said. 

Severe winter weather in the Pacific Northwest is having a significant impact on Alaska’s operations, the airline told Fortune. So far, Alaska canceled more than 248 flights from Seattle on Sunday and 120 flights throughout Monday. 

Delta said Monday that it expects to cancel 200 of 4,166 scheduled mainline and Delta Connection flights thanks to winter weather impacting its hubs in Minneapolis–St. Paul, Seattle, and Salt Lake City (SLC); the Omicron variant continues to hamper Delta’s operations.

End in sight

As Omicron works its way through the general population, it’s spreading among U.S. flight crews as well, says Helane Becker, an airline analyst at securities firm Cowen. The fact that crews don’t want to get stuck somewhere—quarantine rules currently stipulate that personnel who test positive need to limit contact for 10 days—is also part of the problem. 

But that may be shifting. Delta asked for the quarantine time frame to be shortened to five days, which Becker says could alleviate some of the problem. Changing planes could also ease the shortages.

“We think the short term will be beset by strong demand and high cancellations, but the airlines will do their best to combine flights and use larger aircraft in place of smaller aircraft,” Becker told Fortune, estimating between 5% and 10% of airline operations will be affected. 

After the holiday travel period ends next week, Becker says the staffing crunch should ease up as fewer Americans are expected to travel. Becker estimates the downturn will last through mid-February before travel picks back up with an upturn that lasts through the summer months. 

Charlie Leocha, president of Travelers United, a nonprofit that focuses on consumer issues with travel, agrees flight cancellations and delays will likely ease after New Year’s. “The big issues with the cancellations and the inability of the airlines to move people around are going to drop a little bit after the holidays,” he says. “Right now most flights are going out full or at least around 90%. So the situation we’re in right now with that has led to a lot of the delays,” Leocha says. “The abandonment of vacations and people staying home has not really come to fruition.”

Business travel, however, won’t come back until two to four weeks after return to office, according to Becker. But she says the U.S. will likely be within 80% of pre-pandemic levels by year-end 2022.

Currently, the seven-day average of new daily COVID cases in the U.S. is 184,802, according to Johns Hopkins University & Medicine. But Francis Collins, director of the National Institutes of Health, recently warned the U.S. could soon see 1 million new cases per day thanks to the Omicron variant. In a week, Omicron went from accounting for 12.6% of all reported U.S. cases of COVID-19 to 73.2% as of Dec. 18, according to the Centers for Disease Control and Prevention.