Due to Omicron-related disruptions, almost 4,000 flights were cancelled on Sunday.

On Sunday, more than 4,000 flights were cancelled throughout the world, with more than half of them in the United States, adding to the toll of Christmas travel disruptions caused by bad weather and the Omicron variant's spike in coronavirus cases.

Nearly 2,400 flights entering, departing from, or within the United States had been cancelled by 8 p.m. GMT on Sunday, according to flight tracking website FlightAware.com.  Over 11,200 flights were cancelled around the world.

SkyWest and SouthWest were the airlines with the highest cancellations, with 510 and 419 cancellations, respectively, according to FlightAware.

The holidays are always a busy time for air travel, but the quick spread of the highly transmissible Omicron form has resulted in a spike in COVID-19 cases, prompting airlines to delay flights while pilots and cabin personnel quarantine.

Transportation agencies around the country were either stopping or decreasing services due to staff shortages caused by the coronavirus.

In many parts of the world, Omicron has resulted in record case counts and dampened New Year's celebrations.

The rise of COVID instances in the United States  had caused some organisations have decided to rethink their plans to expand the number of employees working from their offices starting Monday.

At least 346,869 new coronavirus cases were reported in the United States on Saturday, according to a Reuters tally.  COVID-19 has killed at least 377 people in the United States, bringing the total to 828,562.

Despite generous financial incentives, American airline cabin workers, pilots, and support employees were hesitant to work overtime during the holidays. According to certain airline unions, many people were afraid of getting COVID-19 and didn't want to deal with disruptive passengers.

Airlines were recruiting employees in the months leading up to the holidays to maintain stable employment after furloughing or laying off hundreds in the last 18 months as the pandemic crippled the industry.