Thousands of flights are canceled as Omicron thins crews.

On the other hand, airlines also expect lots of travel on Jan. 2, a Sunday. And the Omicron variant, which is now responsible for more than 70 percent of the new coronavirus cases in the United States, has already helped push daily case averages in the United States above 200,000 for the first time in nearly 12 months, according to The New York Times’s coronavirus tracker.

An airline trade group has asked the Centers for Disease Control and Prevention to shorten the recommended isolation period for fully vaccinated employees who test positive to a maximum of five days, from 10 days, before they can return with a negative test.

“Swift and safe adjustments by the C.D.C. would alleviate at least some of the staffing pressures and set up airlines to help millions of travelers returning from their holidays,” said Derek Dombrowski, a JetBlue spokesman.

The flight attendants’ union, however, has argued that reductions in recommended isolation times should be decided on “by public health professionals, not airlines.”

Some of this weekend’s delays had little to do with the pandemic. Alaska Airlines had instituted an extensive program to keep crews healthy and even had members of its management team who are trained to be crew members step in, said a spokeswoman, Alexa Rudin.

On Saturday and Sunday, it had only a handful of cancellations related to crew exposures to the coronavirus, according to Ms. Rudin. Yet it had canceled 170 flights those two days, according to FlightAware, including 21 percent of its Sunday flights, because of unusually cold and snowy weather in the Pacific Northwest, which affected its hub, Seattle-Tacoma International Airport.

“Snow and wintry conditions are creating a bit of a bah-humbug for our operations to and from Seattle,” the company said in a statement Sunday, adding, “We realize it’s incredibly frustrating when travel doesn’t go as planned.”