United Airlines and Delta Air Lines have canceled dozens of flights initially scheduled to take off on Christmas Eve and Christmas Day, citing a spike in Omicron COVID-19 infections. (Reuters)

America’s United Airlines and Delta Air Lines have cancelled dozens of Christmas Eve flights, with COVID-19 Omicron variant taking a heavy toll on flight crews and other workers.

Chicago-based United Airlines, which operates a large domestic and international route network, cancelled 120 flights for Friday, while Atlanta-based Delta, one of the major US airlines, said it has cancelled about 90 flights.

The nationwide spike in Omicron cases this week has had a direct impact on our flight crews and the people who run our operation. As a result, we’ve unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport, United Airlines said in a statement on Thursday.

Delta Air Lines, in a separate statement, said it has exhausted all options and resources -- including rerouting and substitutions of aircraft and crews to cover scheduled flying -- before cancelling around 90 flights for Friday.

It cited potential inclement weather and the impact of the Omicron variant for the cancellations.

On Tuesday, Delta Air Lines chief executive Ed Bastian had asked the head of the US Centers for Disease Control and Prevention (CDC) to shrink quarantine guidelines for fully vaccinated individuals who experience breakthrough COVID-19 infections, citing the impact on the carrier’s workforce.

Bastian asked that the isolation period be reduced to five days from the current 10 days.

Airlines for America (A4A), a trade group representing major cargo and passenger carriers, also made a similar request wrote in a letter to the CDC on Thursday.

The Omicron surge may exacerbate personnel shortages and create significant disruptions to our workforce and operations, wrote A4A CEO, Nick Calio.

Much has changed since the initial guidance was developed and issued in 2020 and we believe that variables such as vaccine rates, improved treatments and mask mandates should be considered as the pandemic and science continue to evolve.

The CDC issued new quarantine guidance for healthcare workers on Thursday, reducing the isolation time to seven days for workers who test positive for COVID-19 but are asymptomatic, provided they test negative.

The omicron variant of the coronavirus has quickly overtaken delta as the dominant strain of the virus across the US, accounting for 90 percent of the cases in some parts of the country, CDC director Dr. Rochelle Walensky said Wednesday.

The variant makes up more than 73 percent of the cases in the US, according to the latest data released by the CDC. Last week, US health officials said omicron accounted for 2.9 percent of all cases sequenced through Dec. 11, but later revised that number up to 12.6 percent.

Unvaccinated people are 10 times more likely to contract the virus and 20 times more likely to die from it when compared to a person who is fully vaccinated and received their booster, according to CDC data.

According to reports, there are around 50 mutations in the omicron variant compared to the original SARS-CoV-2 virus detected in China in 2019.