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.

Dr. Anthony Fauci, the top U.S. infectious disease expert, said Monday the nation should consider a vaccination mandate for domestic air travel, signaling a potential embrace of an idea the Biden administration has previously eschewed, as Covid-19 cases spike.

Fauci, President Joe Biden’s chief science adviser on the pandemic response, said such a mandate might drive up the nation’s lagging vaccination rate as well as confer stronger protection on flights, for which federal regulations require all those age 2 and older to wear a mask.

“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.”

The Biden administration has thus far balked at imposing a vaccination requirement for domestic air travel. Two officials said Biden’s science advisers have yet to make a formal recommendation for such a requirement to the president.

The officials, who spoke on condition of anonymity to discuss internal deliberations, said a vaccine mandate on planes could trigger a host of logistical and legal concerns.