Commercial airlines around the world canceled more than 4,000 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 2,314 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.

The website showed that another 1,404 Christmas Day flights were called off worldwide, along with 340 more that had been scheduled for Sunday.

Commercial air traffic within the US and into or out of the country accounted for roughly a fourth 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.

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

The average number of new COVID 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.

Rising hospitalizations were hitting healthcare systems especially hard in the US 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.

One in 20 Londoners had COVID-19 last week, a figure that could rise to one in 10 by early next week, according to data released on Thursday by the Office for National Statistics.

Government data showed a record tally of 122,186 new infections nationwide on Friday, marking a third day in which the number of known cases has surpassed 100,000.

While recent research suggests Omicron produces milder illness, and a lower rate of hospitalizations, than previous variants of COVID-19, health officials have maintained a cautious note about the outlook.