U.S. airlines got a much needed demand boost over the Thanksgiving holiday weekend, posting some of their busiest days since before the coronavirus pandemic began as travelers looked to reunite in-person with their families after hosting virtual gatherings last year. However, airlines still face more threats to their overall recovery as a new coronavirus variant has been discovered in multiple countries.

The Transportation Security Administration (TSA) screened more than 2.45 million passengers on Sunday, according to data compiled by the TSA, topping the year prior's total of about 1.2 million and recording the most travelers in a single day since Feb. 2020, or before the coronavirus was declared a pandemic by the World Health Organization (WHO).

In total, the holiday travel period--between Tuesday to Sunday--screened more than 12 million passengers this year, according to the TSA, compared to the more than 4 million during the same period last year.

Despite the surge in air travel over the holiday, airlines and the overall travel industry still face an uphill battle towards the sector's overall recovery; international travel now is threatened again as dozens of countries report cases of the newly discovered Omicron variant of the virus.

The Omicron variant, which was first discovered by scientists in South Africa, has raised concerns among international health officials and governments over whether it could be more transmissible than the original strain of the virus.

The WHO warned in its risk assessment published Sunday that the Omicron variant poses a global health risks.

"Given mutations that may confer immune escape potential and possibly transmissibility advantage, the likelihood of potential further spread of Omicron at the global level is high," WHO scientists wrote. "Depending on these characteristics, there could be future surges of COVID-19, which could have severe consequences, depending on a number of factors including where surges may take place. The overall global risk related to the new [variant of concern] Omicron is assessed as very high."

However, President Joe Biden on Monday said that he does not expect the United States to impose additional travel restrictions in response to the Omicron variant, only barring travelers from South Africa, Botswana, Simbabwe, Namibia, Lesotho, Eswatini, Mozambique, and Malawi at this time.

The new travel rules come only three weeks after the U.S. lifted its strict coronavirus-era restrictions that barred entry to foreign visitors from more than 30 nations.