Flight disruptions in the United States continued on Monday as many people made their first trip in nearly two years, and Dr.Anthony S. Fauci, the nation’s foremost infectious disease expert, said again raised the possibility of a vaccination requirement for air travel.

At least 2,600 more flights were canceled on Monday, including around 1,000 US flights, as the highly transmissible Omicron variant of the coronavirus sends daily workloads to parts of the United States reaching levels above the winter pandemic peak last.

While cancellations made up only a small percentage of all flights, the problem threatened to spread to the vacation week.

“When you make vaccination a requirement, it is another incentive to get more people vaccinated,” Dr Fauci told MSNBC on Monday. “If you want to do this with domestic flights, I think this is something that should be seriously considered.”

Over the holiday weekend, airlines canceled thousands of flights as the Omicron variant hit flight crews. In total, about 2,300 US flights were canceled on Saturday and Sunday over Christmas weekend, with more than 3,500 more on the ground around the world, according to FlightAware, which provides aviation data. As of Sunday alone, more than 1,300 US flights and nearly 1,700 additional flights around the world were canceled.

While some of the groundings were caused by bad weather and maintenance issues, several airlines acknowledged that the current wave of coronavirus cases had contributed significantly. A spokesperson for JetBlue said the airline had “seen an increasing number of Omicron sickness calls.”

As of Sunday, 12% of JetBlue flights, 6% of Delta Air Lines flights, 5% of United Airlines flights and 2% of American Airlines flights were canceled, according to FlightAware.

Stock prices of United, Delta, American and Southwest – the four largest US carriers – were slightly lower on Monday.

Travel has rebounded sharply this year, worsening the situation at airports: About two million people passed through checkpoints every day last week, according to the Transportation Security Administration, and on Sunday. The numbers on Christmas Eve and Christmas Day were much higher than last year, and some numbers even exceeded those of the same days two years ago, when virtually no American was aware that a virus was starting. to travel to the other side of the world.

The Omicron variant, which is now responsible for more than 70 percent of new coronavirus cases in the United States, has already helped push daily US case averages above 200,000 for the first time in close of 12 months, according to The New York Times coronavirus tracker.

A business group of airlines 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, instead of 10 days, before they can return with a negative test.

“Quick and secure adjustments by the CDC would ease at least some of the staffing pressures and set up airlines to help millions of travelers returning from vacation,” said Derek Dombrowski, a spokesperson for JetBlue. .

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

Some of the delays this weekend had little to do with the pandemic. Alaska Airlines has had only a few cancellations related to the crew’s exposure to the coronavirus, spokeswoman Alexa Rudin said. Yet it canceled 170 flights in the two days, according to FlightAware, including 21% of its Sunday flights, due to unusually cold and snowy weather in the Pacific Northwest, which affected its hub, the international airport of Seattle-Tacoma.

The pandemic has also caused a shortage of train and bus workers nationwide. In New York City, the Metropolitan Transportation Authority is also facing a slight increase in positive cases among its staff, who are 80 percent vaccinated. He said Monday’s metro service was operating on a regular schedule, with a few exceptions.

“Anything we can do as passengers to help minimize the risk to transit workers will help reduce the spread,” said Lisa Daglian, executive director of the Citizens’ Standing Advisory Committee to the MTA, a group monitoring. “The MTA is doing what it can with the resources it has. “

Danny Pearlstein, spokesperson for the Riders Alliance, an advocacy group, said: “I feel like the MTA is making the most of a bad situation again.”