Commercial airlines around the world canceled more than 4,500 flights over the Christmas weekend, as a mounting wave of COVID-19 infections driven by the Omicron variant created greater uncertainty and misery for holiday travelers.

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.

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 U.S. carriers to report a wave of holiday weekend cancellations were United Airlines and Delta Air Lines (DAL.N), which scrubbed nearly 280 flights combined on Friday alone, citing personnel shortages amid the surge of COVID-19 infections.

COVID-19 infections have surged in the United States in recent days due to the highly transmissible variant Omicron, which was first detected in November and now accounts for nearly three-quarters of U.S. cases and as many as 90% in some areas, such as the Eastern Seaboard.

The average number of new U.S. coronavirus cases has risen 45% to 179,000 per day over the past week, according to a Reuters tally.

New York reported more than 44,000 newly confirmed infections on Friday alone, shattering that state’s daily record. At least 10 other states set new one-day case records on Thursday or Friday.

Rising hospitalizations were hitting healthcare systems especially hard in the U.S. Midwest, with intensive care units in Indiana, Ohio and Michigan bracing for the worst even as they remain under pressure from an earlier wave of Delta variant cases.

In Britain, many industries and transport networks were struggling with staff shortages as sick workers self-isolated, while hospitals have warned of the risk of an impact on patient safety.