Elevated numbers of flight cancellations were stretching into a second week as carriers scrambled to get travelers to their destinations amid a coronavirus spike that has led to staffing shortages and weather that has slowed operations.

As of Thursday evening, 1,300 flights within, into and out of the United States had been canceled for the day, according to flight tracking website FlightAware. More than 700 flights scheduled for Friday already had been canceled, with more than 500 others cut on Saturday.

As cancellations extended days into the future, airlines and passengers grappled with the likelihood of disruptions days into the new year. While airlines have been hit hard since before Christmas, traveler delays were extending beyond airports to include rail networks and transit agencies.

In a Federal Aviation Administration statement issued Thursday – with the words “could” and “might” underlined and in bold – the agency warned of the potential for disruptions in the coming days. The FAA said it also is facing staffing shortages because of rising coronavirus infections.

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“Weather and heavy seasonal traffic are likely to result in some travel delays in the coming days,” the agency said. “Like the rest of the U.S. population, an increased number of FAA employees have tested positive for COVID-19.”

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The rising number of cases also triggered disruptions for Amtrak, which said Thursday it was canceling about two dozen trains between Dec. 31 and Jan. 6 in both the Northeast Corridor and along its long-distance routes – about 1.5% of scheduled service during that time.

“We are continuing to monitor changing conditions and will make any further adjustments as required,” the railroad said in a statement, adding that 97% of its employees are vaccinated.

Staffing shortages also were hitting public transportation agencies.

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At the nation’s airports, concerns arose about whether carriers could cope with passenger volumes during the busy holidays after high-profile breakdowns over the summer and fall. Airlines passed their first major test over Thanksgiving, but Christmas has proved to be a different story, leaving thousands of frustrated air travelers unable to return home.

“This is a no-win situation for everyone,” said Henry Harteveldt, an aviation analyst and president of Atmosphere Research. “Airlines have lost this week. Passengers have lost this week. And it’s made worse by the fact it’s happening at Christmastime and the New Year’s break.”

While cancellations Thursday were widespread, United Airlines remained the hardest-hit among major carriers, with 198 flights canceled, about 9% of its schedule. JetBlue, which announced Wednesday it would reduce flights through Jan. 13, had 175 flights canceled. Regional carrier SkyWest also continued to be plagued by operational difficulties with 9% of its scheduled flights canceled.

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Delta said it expected to cancel about 250 of more than 4,000 Delta and Delta Connection flights Thursday. The airline said cancellations will probably continue through the weekend with 200 to 300 daily cancellations.

“Delta people are continuing to work together around the clock to reroute and substitute aircraft and crews to get customers where they need to be as quickly and safely as possible,” the airline said in a statement.

Delta issued travel waivers for customers whose itineraries include Chicago, Detroit, Salt Lake City and Seattle – where storms are expected to complicate travel – and urged travelers whose plans include those cities to reschedule.

Kerry Tan, an associate professor of economics and an air travel expert at Maryland’s Loyola University, said that while weather is often a factor at this time of year, staffing issues have proved to be a greater challenge for carriers.

“The weather aspect is out of their hands,” Tan said. “What they can control is staffing, but like many companies out there, there are huge staffing issues. There are just not enough workers to meet the demand.”

Outside of the period following the Sept. 11 attacks, few in the industry could recall a time when so many flights had been affected for such a long duration.

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“It’s the perfect storm,” said Dennis Tajer, a spokesman for the Allied Pilots Association, which represents crews at American Airlines.

Harteveldt noted that disruptions are not unique to the United States. According to FlightAware, more than 2,800 flights worldwide were canceled Thursday.

“We’re seeing airlines affected in Europe, Africa, Asia and elsewhere, so this is truly a global airline industry challenge,” he said.

The first signs of trouble emerged just before Christmas when airlines, citing staffing issues resulting from the more easily transmissible omicron variant of the coronavirus, began preemptively canceling flights. On Christmas Eve, about 613 flights were canceled, according to FlightAware. The day after Christmas, the number had ballooned to more than 1,400. Airlines have canceled nearly 8,000 flights in recent days.

In a shift that could help ease staffing shortages, the Centers for Disease Control and Prevention this week updated its guidance on the isolation period for those who test positive for the coronavirus, saying they need to isolate for only five days rather than 10.

CDC Director Rochelle Walensky said data indicates that most transmission occurs early in the course of a person’s illness, typically in the one to two days before symptoms appear and two to three days afterward. Health officials also recommended that those exposed get tested five days after their exposure.

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Delta Air Lines quickly embraced the new guidelines, which chief executive Ed Bastian had urged the CDC to update in a letter to Walensky last week.

The carrier said that starting Monday, it will limit pay protection for vaccinated employees who test positive to five days, according to a memo to employees, with an additional two days for anyone who tests positive on the fifth day. The airline had previously offered 10 days of pay.

The memo ties the change to new CDC guidance on isolation. The airline encouraged employees to take either a rapid or PCR test on their fifth day of isolation. The CDC guidance applies to asymptomatic people and those who are fever-free for 24 hours. The memo makes no mention of what employees should do if they are symptomatic.

Southwest Airlines and other carriers said they are evaluating the CDC’s updated guidance.

The CDC’s shift continued to generate concern among some labor groups.

In a letter to executives at Alaska Airlines, Frontier Airlines, Hawaiian Airlines, United and other carriers, Sara Nelson, international president of the Association of Flight Attendants – which represents crew members at 17 airlines – urged carriers to maintain the 10-day quarantine recommendation as a demonstration of “your commitment to safety and the safety of those working on the front lines.”

If carriers decide to embrace the updated recommendations, Nelson urged them to include other safeguards, including proof of a negative coronavirus test at the end of the five-day isolation period. Wrote Nelson: “No one should feel pressured to come to work sick.”