U.S. Airlines Cancel Flights as Omicron Disruptions Grow

United Airlines Holdings Inc.

is canceling dozens of flights over the holiday weekend as a surge of Covid-19 cases affects crews.

Delta Air Lines Inc.

also cited the Omicron variant as a factor behind a number of cancellations.

At both airlines, the cancellations account for a relatively small share of planned flying. So far, United has canceled about 136 flights scheduled for Friday, about 7% of its planned schedule, and about 28 that were slated for Saturday, according to FlightAware, a flight-tracking site.

In the U.K., the head of the Royal College of Nursing, Pat Cullen, told the British Broadcasting Corp. that the National Health Service is struggling with staff absences as Omicron spreads. The U.K. Health Security Agency’s chief has indicated that the British government might decide whether to introduce more restrictions in England by assessing the wider social impact of the infection, rather than the severity of the disease itself.

Prime Minister

Boris Johnson

has said his government won’t introduce further restrictions before Christmas, but it is possible more measures will be introduced next week. Scotland, Wales and Northern Ireland have already announced wider social restrictions after Christmas.

A U.K. study released Thursday found that people infected with Omicron are between 50% to 70% less likely to be hospitalized than those who caught earlier strains. The finding adds to a growing body of evidence that its effects are less severe in populations with a high level of immunity. The U.K. Health Security Agency study follows similar findings from studies in Scotland and South Africa that also pointed to a substantially lower risk of hospitalization with Omicron than with earlier variants.

The swift advance of the variant has led public-health experts to say that Covid-19 is now on the path toward becoming endemic in the U.S., eventually dissipating into something like a regular seasonal illness. The level of disruption it causes will likely now depend on what level of disease—and restrictions—officials and individuals are willing to tolerate.

“It is a tug of war between society and the virus,” said Peter Chin-Hong, an infectious-disease specialist at the University of California, San Francisco.

Already, more health experts, business people and government officials are assessing how long people infected with Covid-19 should isolate if they are vaccinated and no longer testing positive, suggesting that responses to the pandemic are now shifting.

The U.S. Centers for Disease Control and Prevention has recommended that people with Covid-19 should isolate for 10 days from the first day symptoms develop or from a positive test, to prevent spreading the virus. On Thursday, the agency issued new guidelines for healthcare workers, reducing their recommended isolation time.

The U.K. changed its quarantine rules on Wednesday, cutting the quarantine time to seven days for vaccinated people who twice test negative, to allow them to return to work faster, easing the burden on overstretched public services and businesses, the government said.

Elsewhere, Thailand detected its first domestic cluster of Omicron transmissions, in Kalasin province, north of Bangkok. Bangkok also canceled city-led New Year celebrations, including midnight prayers that are typically held by thousands of Buddhist monks. In New York City, Mayor

Bill de Blasio

said on Thursday that attendance at the annual New Year’s Eve celebration in Times Square will be limited to 15,000 compared with the typical 58,000. 

Austria has joined a other countries now considering a fourth vaccination, which will be offered to healthcare workers and other key employees if it is approved. Israel has already made plans to offer a fourth shot to the over-60s while Germany is considering a similar course of action.

Also in Europe, the Spanish government this week reintroduced an outdoor mask mandate amid a surge in infections. The Italian government did likewise on Thursday, the same day the country reported its highest number of daily infections since the start of the pandemic.

Write to James Hookway at james.hookway@wsj.com