The White House on Thursday said a COVID-19 vaccine requirement for domestic travel is still on the table as the United States heads into the winter months and battles the new omicron variant, saying the Biden administration will continue to "evaluate and assess on a daily basis."

President Biden on Thursday is set to formally announce a series of steps to take on COVID-19, including a requirement for all international passengers flying to the U.S. to test for COVID within 24 hours of departure – regardless of vaccination status – and an extension of its mask requirement on all domestic flights and public transportation.

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White House press secretary Jen Psaki on Thursday was asked whether a vaccine requirement would be added for passengers traveling on domestic flights.

White House press secretary Jen Psaki speaks during a daily news briefing at the White House on Dec. 2, 2021, in Washington.

White House press secretary Jen Psaki speaks during a daily news briefing at the White House on Dec. 2, 2021, in Washington. (Anna Moneymaker/Getty Images)

"I would say that nothing is off the table, so, including domestic travel," Psaki said, noting that there are "some strong protections in place already, including the requirement of mask wearing."

The White House is set to extend the mask mandate to March 18, adding a doubling of fines if individuals are noncompliant with the requirement. The current mask mandate was slated to expire on Jan. 18 following an extension last August.

The mask requirement has contributed to a spike in altercations on flights and other forms of public transportation. Last month, Attorney General Merrick Garland ordered the Justice Department to enhance its efforts to prosecute unruly passengers.

"We base our decisions on the advice of the health and medical experts," Psaki said, noting that officials are looking at "a range of factors as we make decisions about what steps we can put in place." 

"Again, nothing’s off the table," Psaki added.

Holiday travelers walk through the Miami International Airport on Nov. 23, 2021.

Holiday travelers walk through the Miami International Airport on Nov. 23, 2021. (AP Photo/Marta Lavandier)

And White House coronavirus task force coordinator Jeffrey Zients this week was asked during an interview on MSNBC about a potential domestic flight vaccine requirement –  something he, too, left as an option.

"We do have those tight requirements on the international travel, we have the workplace requirements, we’ll continue to look at all options and everything’s on the table," Zients said.

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The changes in travel and mask requirements come as public health officials have expressed concern about the omicron variant, the latest mutant strain of COVID-19.

Biden, last week, restricted travel from South Africa and seven other countries over concerns of the "heavily mutated" omicron variant of COVID-19 – a move he said Monday gives the United States "time." 

As for travel bans from other nations reporting cases of the omicron variant, the White House said it is continuing to "assess on a daily basis," and that the president will "always err on the side of protecting the American people." 

"The decision to put in specific travel restrictions from a set of countries was made based on the recommendation of our health and medical team and based on the fact that there were hundreds, if not thousands, of cases that we were seeing in South Africa and concerned about in neighboring countries," Psaki said. "So that was a decision that was made at that time – no one wants that to be permanent." 

She added that the administration is assessing whether "additional restrictions need to be put in place."

But this week, a California resident who was vaccinated against COVID-19 was confirmed as the first U.S. case of the omicron variant.

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The Centers for Disease Control and Prevention (CDC) on Thursday also identified a second case of the variant in the United States – a Minnesota resident who had traveled to New York.

As for the threat that the omicron poses, Psaki on Thursday said there is "a lot we don’t know."

"And that is challenging, and frustrating, I understand, for Americans, for all of you," Psaki said. "And we want to give the space and time for the public health officials who are working 24/7 to assess and provide guidance on it."

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She added: "It could be less deadly, it could be more. We don’t know."

Meanwhile, the White House stressed that the new measures are implemented based on what officials believe is "going to be most effective." 

"We've continued to add and make vaccines and boosters more available and accessible," Psaki said. "So we will continue to evaluate and assess on a daily basis."