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Two major airlines canceled flights the day before Christmas Eve, siting complications including inclement weather and the omicron variant of COVID-19.

“Delta teams have exhausted all options and resources — including rerouting and substitutions of aircraft and crews to cover scheduled flying — before canceling around 90 flights,” a statement sent to PIX11 News said.

Delta did say it still plans on running 3,100 flights Friday. The flights that are canceled were canceled “due to a combination of issues,” Delta said in a statement.

“The nationwide spike in omicron cases this week has had a direct impact on our flight crews and the people who run our operation,” United said in a statement to several news outlets. “As a result, we’ve unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport.”

The airline said it was working to rebook as many people as possible.

Similarly, United canceled more than 100 Friday flights, according to flight tracking site FlightAware.

The cancellations come as coronavirus infections fueled by the new variant further squeeze staffing at hospitals, police departments, supermarkets and other critical operations struggling to maintain a full contingent of front-line workers.

To ease staffing shortages, countries including Spain and the U.K. have reduced the length of COVID-19 quarantines by letting people return to work sooner after testing positive or being exposed to the virus.

Delta CEO Ed Bastian was among those who have called on the Biden administration to take similar steps or risk further disruptions in air travel. On Thursday, the U.S. shortened COVID-19 isolation rules for health care workers only.

Associated Press contributed to this report.