American Airlines won’t require passengers to prove they’re vaccinated because it would be “incredibly cumbersome” to do that inside the U.S., the airline’s CEO told the New York Times.

Chief executive Doug Parker said checking passengers for proof of vaccination “wouldn’t be physically possible” on domestic flights without causing enormous delays to the airline system. By contrast, this could be done on international flights because there is more time between flights and passengers are already checked for passports, he noted.

“In a world where we still have whatever it is, 40% of the United States not vaccinated for reasons only they can explain, requiring vaccinations to travel and not requiring vaccinations to do anything else around the country isn’t something we’re looking to do,” Parker added.

Parker, who is also the airline’s chairman, made the comments during an interview on the Times’ “Sway” podcast. American Airlines controls roughly 70% of air traffic at Philadelphia International Airport.

The airline isn’t requiring its employees to get vaccinated either, unlike other companies. Parker said the firm offers incentives to staffers who get the shots, such as an extra day of vacation and a $50 gift card.

“We certainly encourage it everywhere we can, encourage it for our customers and our employees, but we’re not putting mandates in place,” Parker told the Times.

Parker also discussed the uptick in unruly passenger behavior. American Airlines now receives about 100 “customer misconduct reports” a day, up from about 30 in 2019, Parker said. Serious events like assaults that require action against the customer are up too, he said. Still, those events reflect about 1 in 300,000 customers, according to Parker.

“But we fly 600,000 people a day, so that’s two people a day that we’re having to take action against,” Parker said. “It’s incredibly frustrating to me, because we’ve worked so hard to make sure it doesn’t continue, and we have been for months. But it still is happening, which I don’t fully understand.”