Thousands of would-be travelers received the same troubling message on Thursday: a last-minute cancellation of their Christmas flights on Friday and Saturday because of the recent spike of Omicron cases, including among airline workers.

The number of cancellations globally as of Friday morning added up to more than 3,000, the Flight Aware website showed. It was the latest blow to the holiday season, mainly caused by the new and highly transmissible Omicron variant, which now accounts for more than 70 percent of new coronavirus cases in the United States.

Delta Air Lines said that it had canceled about 135 flights for Friday after exhausting “all options and resources,” including rerouting and substituting planes and crews to cover scheduled flights. It attributed the cancellations to “a combination of issues, including but not limited to, potential inclement weather in some areas and the impact of the Omicron variant.”

United Airlines canceled at least 150 flights scheduled to leave dozens of airports on Friday — along with 44 more that were supposed to take off on Saturday, according to Flight Aware. Other airlines, including JetBlue and Allegiant, did likewise, although American Airlines said on Friday morning that it currently had no flight cancellations.

In Australia, dozens of flights were canceled at airports in the major cities of Sydney and Melbourne as coronavirus cases in the country surged to their highest since the start of the pandemic. And in Europe, a spokeswoman for the Eurostar train service said on Friday morning that because of travel restrictions across the continent, a small number of trains had been canceled amid a drop in demand.

The United States is recording nearly 170,000 new daily cases, a 38 percent increase over the last two weeks, according to The New York Times’s coronavirus tracker.

United said in a statement that Omicron’s “direct impact on our flight crews and the people who run our operation” had led to the cancellations. Crew members have been calling in sick, according to a spokesman, Joshua Freed, who said United had alerted customers as soon as it was able to. And while Mr. Freed said he did not expect the airline to cancel more flights, it remained a possibility.

“We are really managing this day by day,” he said. “There may be some more flight cancellations for Saturday. It’s possible.”

Customers took to social media to air their grievances about the cancellations.

In Australia, which has recorded more than 500 Omicron cases, many airline staff members are unable to work after being identified as close contacts of positive coronavirus cases, airline officials said. Under government requirements, they are required to isolate for seven days.

“A large number of our frontline team members are being required to test and isolate as close contacts given the increasing number of cases in the general community,” a representative for Jetstar Airways said by email on Friday. “As a result, we have had to make some late adjustments to our schedule.”

Staffing shortages across various sectors have been affecting services in many countries as the virus continues to spread.

England said this week that it was reducing the number of days that people must isolate for after showing Covid-19 symptoms to seven days from 10 days, a change that officials said could help alleviate the shortages. The U.S. Centers for Disease Control and Prevention made a similar move Thousands of would-be travelers received the same troubling message on Thursday: a last-minute cancellation of their Christmas flights on Friday and Saturday because of the recent spike of Omicron cases, including among airline workers.

The United States is recording nearly 170,000 new daily cases, a 38 percent increase over the last two weeks, according to The New York Times’s coronavirus tracker.

“We are really managing this day by day,” he said. “There may be some more flight cancellations for Saturday. It’s possible.”

Customers took to social media to air their grievances about the cancellations.

Staffing shortages across various sectors have been affecting services in many countries as the virus continues to spread.

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