Dr. Anthony Fauci on Monday said he thought the U.S. should consider instituting a mandate that would require domestic air travelers to be vaccinated against COVID-19.

During an appearance on MSNBC's "Morning Joe," Fauci — a White House medical adviser — declined to say whether he had briefed President Joe Biden on such a policy. However, he did say a vaccine requirement to board domestic flights is "reasonable to consider."

"There's requirements whether you want to get into college or a university, if you want to work in certain places, and if you make vaccinations a requirement that's another incentive to get more people vaccinated," Fauci said. "If you want to do that with domestic flights, I think that's something that should be considered."

Fauci's comments come as hundreds of flights across the country were delayed or canceled as airlines experienced worker shortages amid COVID-19 outbreaks.

The current spike in COVID-19 cases is largely being driven by the omicron variant — a highly contagious strain that now represents at least 73% of all new cases across the U.S.

"We're in a tough situation with omicron. It's not something to be taken lightly," Fauci said.

Fauci noted that data from South Africa — one of the first countries to experience an omicron surge — indicate that the new strain may not cause as serious cases of COVID-19 as other strains.

While it's unclear how the virus will behave in the U.S., Fauci noted that officials are considering lowering the time of isolation from a COVID-19 diagnosis from 10 days to seven days to prevent further worker shortages in essential industries.

"Certainly, with the health care workers, we're going to be doing it," Fauci said, noting that the administration will consider lowering the isolation period for workers in other industries deemed "essential."

In September, Fauci told theSkimm Podcast that he would support a vaccine mandate for air travel.

According to the Centers for Disease Control and Prevention, the U.S. saw its second-highest day of new infections last week, with nearly 300,000 new cases on Monday alone. The current seven-day average of new cases is about 175,000 a day, up from about 118,000 on Dec. 14. During that same period, deaths have remained fairly stagnant, rising from an average of 1,100 per day to 1,200 per day.