United Airlines CEO Scott Kirby is advocating for all airlines to adopt a policy that would mandate COVID-19 vaccines for all employees.

“The worst thing that I believe I will ever do in my career is the letters that I have written to the surviving family members of coworkers that we have lost to the coronavirus,” CEO Scott Kirby said at an employee town hall Thursday, a transcript of which was reviewed by CNBC . “And so, for me, because I have confidence in the safety of the vaccine – and I recognize it’s controversial – I think the right thing to do is for United Airlines, and for other companies, to require the vaccines and to make them mandatory.”

As Kirby said, it’s a controversial statement to be sure. Airline employees are considered essential workers and can get the vaccine long before the general public, but there is a large fraction of people – no matter what industry they are in – reluctant to take a vaccine.

“I don’t think United will get away with and can realistically be the only company that requires vaccines and makes them mandatory,” Kirby said. “We need some others. We need some others to show leadership. Particularly in the healthcare industry.”

In the employee memo, Kirby said United is working with government officials and health-care providers to set up vaccine distribution centers at some of its bigger hubs.

“It’s certainly a sensitive topic all the way around,” Michael Klemm, president of the International Association of Machinists and Aerospace Workers District 141, which represents fleet and passenger service workers at United, told CNBC. “We’ve received some frustration from members who don’t want to take the vaccine as well as concern from members who don’t want to work with someone who doesn’t take it.”

“Right now, Flight Attendants are in different tiers for access in each state,” Association of Flight Attendants spokeswoman Taylor Garland said in a statement. “We need a federal approach that prioritizes Flight Attendants as essential workers facilitating interstate commerce.”

Fellow airlines have yet to follow United’s lead. While they have encouraged employees to get the vaccine, they haven’t demanded it.