Airlines’ U.S. flight cancellations approached 2,400 for the Christmas weekend, disrupting travel on one of the busiest periods of the year as the omicron-fueled wave of Covid cases triggered air-crew shortages.

The situation eased a bit Sunday after Saturday’s pullbacks erased at least 12% of the schedule at Delta Air Lines Inc., United Airlines Holdings Inc. and JetBlue Airways Corp., according to data tracker FlightAware.com. JetBlue was the only one of the trio reporting 10% of its schedule canceled on Sunday, with the other two at 5% or less. The U.S. cuts on Christmas Day alone totaled more than 900.

Airlines’ tie-ups added to evidence of economic disruptions from the omicron variant, whose fast spread is causing havoc even though initial indications suggest that it causes less-severe illness than other Covid strains. Health-care systems are being squeezed by a lack of staff, and a U.S. Navy warship is stuck at Guantanamo Bay after a Covid outbreak on board.

“Someone should’ve sounded an alarm when they realized people can get omicron after being vaccinated,” said Henry Harteveldt, president of travel consulting firm Atmosphere Research Group. “It doesn’t seem like they planned well enough.”

The disruptions may extend into the New Year’s holiday weekend, he said. Still, U.S. air travel could face less stress if the federal government shortens the required quarantine for industry workers who test positive with the virus.

The Centers for Disease Control on Thursday reduced isolation time for vaccinated health-care workers who contract the virus. Airlines for America, the trade association representing North American carriers, wrote to the CDC asking for the 10-day quarantine period for fully vaccinated individuals to be cut to “no more than five days.”

Flight cancellations are a “concern at a time when people are traveling to spend time with family and friends for the holidays,” Harteveldt said. “No airline wants to be viewed as the Grinch who stole Christmas.”

Delta’s 612 cancellations for the holiday weekend led the U.S. industry, FlightAware data showed at 11 a.m. New York time Sunday, followed by United at 535. JetBlue had at least 313 flights canceled for the weekend, according to FlightAware.

“Like many businesses and organizations, we have seen an increasing number of sick calls from omicron,” JetBlue spokesperson Derek Dombrowski said in an emailed statement. “Despite our best efforts, we’ve had to cancel a number of flights, and additional flight cancellations and other delays remain a possibility as we see more omicron community spread.”

The holidays are among the year’s peak travel periods. The Transportation Security Administration estimated that Dec. 22 and 23 would be the busiest pre-Christmas travel dates nationally and locally, with Jan. 2 and 3 the most crowded for post-holiday travel.

Consumer-complaint messages piled up on the airlines’ Twitter feeds as passengers lamented the disruptions to their holiday weekend plans. Travelers denounced the flight delays, cancellations and missing luggage on social media.

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

Globally, the tally of dropped flights was about 6,000 trips for Friday through Sunday, according to FlightAware. Cancellations ranked among the highest at Xi’an Xianyang International Airport, in the western Chinese city where the government punished local officials for failing to curb an outbreak that led to the biggest lockdown since Covid emerged in Wuhan. Nearly a third of the flights departing from the airport were canceled Friday and Saturday, according to FlightAware.

Robert Mann, a former American Airlines executive who is now a consultant, said there are about 115,000 flights a day globally during a peak travel period such as Christmas.

“As for China, it’s not a very transparent market over there,” Mann said. “There’s been a number of cities that have shut down, and as result that would be expected to impact air service dramatically.”

Elsewhere, British discounter EasyJet said it had canceled some flights between the U.K. and France and Germany for the rest of December, following the introduction of restrictions on U.K. travelers to those countries.