Dr. Anthony Fauci said Monday that the U.S. should look at implementing a COVID-19 vaccine mandate for people traveling on domestic flights, even though President Joe Biden's administration has been hesitant to do so in the past.

Fauci, the chief science adviser for Biden and director of the National Institute of Allergy and Infectious Diseases, said the federal mandate for air travel should expand to include travelers being vaccinated against COVID-19. The current mandate only requires people age 2 and older to wear a mask on flights.

He suggested the mandate would provide more protection on flights and might help increase COVID-19 vaccination numbers in the country.

White House press secretary Jen Psaki told MSNBC: "We know that masking can be, is, very effective on airplanes." She added that putting in new restrictions might cause further implications or create flight delays.

Biden's administration previously explored the idea of a vaccine mandate with requiring either proof of vaccination or a negative COVID-19 test prior to traveling. However, two anonymous officials said implementing a vaccine mandate could cause legal and logistical concerns.

The two officials also said science advisers have not made a formal recommendation to Biden on whether to implement a vaccine mandate for domestic travels.

Biden has not answered questions on whether or not he plans to implement the vaccine mandate for domestic air travel.

Dr. Anthony Fauci COVID-19 Vaccine Mandate
Dr. Anthony Fauci said Monday he wants to implement a COVID-19 vaccine mandate for domestic travel, but President Joe Biden's administration is still uncertain. Above, Fauci, director of the National Institute of Allergy and Infectious Diseases and the chief medical adviser to the president, gives an update on the Omicron COVID-19 variant during the daily press briefing at the White House on December 1, 2021, in Washington, D.C.Anna Moneymaker/Getty Images

The U.S. currently mandates that most foreign nationals traveling to the U.S. be fully vaccinated against the coronavirus, though citizens and permanent residents only need to show proof of a negative test taken within a day of boarding.

Federal rules don't require people traveling by air within the U.S. to show a negative test. Hawaii requires travelers to test or show proof of vaccination to avoid a mandatory quarantine.

Biden told reporters that the subject regarding the vaccine mandate was discussed on a call with the nation's governors Monday morning.

"They asked Dr. Fauci some more questions about everything from whether or not he thought he was going to move to test at home—I mean, on air flights and that kind of thing," Biden said of the call before departing the White House for his home in Rehoboth Beach, Delaware.

When officials looked at requiring a mandate earlier in the year they were not eager to mandate vaccination for domestic air travel because they expected it to face immediate legal challenges, mitigating its potential effectiveness as a tool to drive up vaccinations.

"We would do it, though, if the health impact was overwhelming. So we rely always on the advice of our health and medical experts. That isn't a step at this point that they had determined we need to take," Psaki said.

Data from the Centers for Disease Control and Prevention show more than 241 million Americans, about 77 percent of the eligible population aged 5 and over, have received at least one shot of a COVID-19 vaccine. Officials believe, though, that there is some over-count in the figures due to record-keeping errors in the administration of booster shots.

"When you make vaccination a requirement, that's another incentive to get more people vaccinated," Fauci told MSNBC. "If you want to do that with domestic flights, I think that's something that seriously should be considered."

Since the summer, the Biden administration has embraced various vaccination requirements as a way to get unvaccinated Americans to roll up their sleeves. It has instituted requirements that federal workers, federal contractors, and those who work in health care get their shots, and that employers with 100 or more employees institute vaccination-or-testing requirements for their workers.

Those vaccination requirements have been mired in legal wrangling, with the Supreme Court set to hear arguments on January 7 in cases seeking to overturn them.

The Associated Press contributed to this report.

Dr. Anthony Fauci Vaccine Domestic Air Travel
Dr. Anthony Fauci, the top U.S. infectious disease expert, said the U.S. should consider a vaccination mandate for domestic air travel as coronavirus infections surge. Above, Fauci wears a face mask as he arrives for the the White House COVID-19 Response Team's regular call with the National Governors Association in the South Court Auditorium in the Eisenhower Executive Office Building on the White House Campus, December 27, 2021, in Washington, D.C. Carolyn Kaster/AP Photo