More canceled flights frustrated air travelers on the final day of 2021 and appeared all but certain to inconvenience hundreds of thousands more over the New Year’s holiday weekend.

Travelers line up for flights at O’Hare International Airport in Chicago on Thursday. Nam Y. Huh/Associated Press

Airlines blamed many of the cancellations on crew shortages related to the spike in COVID-19 infections, along with wintry weather in parts of the United States.

By early afternoon Friday on the East Coast, airlines had scrubbed more than 1,400 U.S. flights – about 6 percent of all scheduled flights – and roughly 2,900 worldwide, according to tracking service FlightAware.

That pushed the total U.S. cancellations since Christmas Eve above 9,000, with the peak of 1,520 on Dec. 26.

The disruptions come just as travel numbers climb higher going into the New Year’s holiday weekend. Since Dec. 16, more than 2 million travelers a day on average have passed through U.S. airport security checkpoints, an increase of nearly 100,000 a day since November and nearly double last December.

Led by United and JetBlue, airlines have already canceled more than 1,000 U.S. flights on Saturday and nearly 500 on Sunday.

Canceled flights began rising from a couple hundred a day shortly before Christmas, most notably for United Airlines, Delta Air Lines and JetBlue Airways.

On Friday, United canceled more than 200 flights, or 11 percent of its schedule – and that doesn’t include cancellations on the United Express regional affiliate. CommutAir, which operates many United Express flights, scrubbed one-third of its schedule by midday, according to FlightAware figures.

JetBlue canceled more than 140 flights, or 14 percent of its schedule, and Delta grounded more than 100, or 5 percent of its flights by midday Friday. Allegiant, Alaska, Spirit and regional carriers SkyWest and Mesa all scrubbed at least 9 percent of their flights.

FlightAware reported fewer cancellations at Southwest, 2 percent, and American, 1 percent.

The virus is also hitting more federal air traffic controllers. The Federal Aviation Administration said that more of its employees have tested positive – it didn’t provide numbers Friday – which could lead controllers to reduce flight volumes and “might result in delays during busy periods.”

While leisure travel within the U.S. has returned to roughly pre-pandemic levels, international travel remains depressed, and the government is giving travelers new cause to reconsider trips abroad. On Thursday, the State Department warned Americans that if they test positive for coronavirus while in a foreign country it could mean a costly quarantine until they test negative.

Since March 2020, U.S. airlines have received $54 billion in federal relief to keep employees on the payroll through the pandemic. Congress barred the airlines from furloughing workers but allowed them to offer incentives to quit or take long leaves of absence – and many did. The airlines have about 9 percent fewer workers than they had two years ago.

Many airlines are now rushing to hire pilots, flight attendants and other workers. In the meantime, some are trimming schedules that they can no longer operate. Southwest did that before the holidays, JetBlue is cutting flights until mid-January, and Hong Kong’s Cathay Pacific is suspending cargo flights and reducing passenger flights because it doesn’t have enough pilots.

Other forms of transportation are also being hammered by the surge in virus cases. The U.S. Centers for Disease Control and Prevention said Thursday that it is monitoring more than 90 cruise ships because of COVID-19 outbreaks. The health agency warned people not to go on cruises, even if they are fully vaccinated against the virus.

The remnants of the delta variant and the rise of the new omicron variant pushed the seven-day rolling average of new daily COVID-19 cases in the U.S. above 350,000, nearly triple the rate of just two weeks ago, according to figures from Johns Hopkins University.