WATCH: Fauci ‘welcomes’ vaccine mandates for domestic flights

Fauci commented that "anything that could get people more vaccinated would be welcome, but stressed wearing masks on planes despite airline executives arguing they're not always necessary due to sufficient air filtration systems inside aircraft.

"Well, it depends on what you want to use it for," Fauci told Karl in the on-air segment. "I mean, vaccine requirements for people coming in from other countries is to prevent newly infected people from getting in to the country."

"A vaccine requirement for a person getting on the plane is just another level of getting people to have a mechanism that would spur them to get vaccinated. Namely, you can't get on a plane unless you're vaccinated, which is just another one of the ways of getting requirements, whatever that might be," he added.

"So, I mean, anything that could get people more vaccinated would be welcome."

Fauci went on to say, "I think the idea of taking masks off, in my mind, is really not something we should even be considering, and that's what we meant by it depends on what the goal of getting people vaccinated before they get on a domestic flight."

With the onset of the Omicron variant, United Airlines moved to cancel dozens of holiday flights scheduled for Friday, Christmas Eve. Southwest Airlines faced criticism back in October over cancelling a thousand flights for "staffing issues." Some pointed to air traffic control, while others blamed vaccine mandates.