Pilots, flights attendants and other staff have been calling in sick or having to quarantine after exposure to Covid-19, forcing Lufthansa, Delta, United Airlines and many other carriers to cancel flights during one of the year's peak travel periods.
Commercial airlines around the world have cancelled more than 4,500 flights over the Christmas weekend.
The development came as mounting wave of Covid-19 infections driven by the Omicron variant created greater uncertainty and misery for holiday travellers.
Airline carriers globally scrapped at least 2,401 flights on Friday, which fell on Christmas Eve and is typically a heavy day for air travel, according to a running tally on the flight-tracking website FlightAware.com.
Nearly 10,000 more flights were delayed.
The website showed that 1,779 Christmas Day flights were called off worldwide, along with 402 more that had been scheduled for Sunday.
United, Delta cancel nearly 300 flights
Commercial air traffic within the United States and into or out of the country accounted for more than a quarter of all the canceled flights over the weekend, FlightAware data showed.
Among the first US carriers to report a wave of holiday weekend cancellations were United Airlines and Delta Air Lines, which scrubbed nearly 280 flights combined on Friday alone, citing personnel shortages amid the surge of Covid-19 infections.
While recent research suggests Omicron produces milder illness, and a lower rate of hospitalisations, than previous variants of Covid-19, health officials have maintained a cautious note about the outlook.