United Airlines announced Friday that it will require all employees to be vaccinated against the coronavirus as a condition of employment, making it the first US major airline to do so and joining a list of other carriers that have mandated the same.

United’s employee mandate will apply to all 67,000 active employees.

“We know some of you will disagree with this decision to require the vaccine for all United employees,” Scott Kirby, United’s chief executive, and Brett Hart, the company’s president, wrote in a letter to employees, according to the Washington Post. “But we have no greater responsibility to you and your colleagues than to ensure your safety when you’re at work, and the facts are crystal clear: everyone is safer when everyone is vaccinated.”

United’s employees’ union, IAM District 141, said employees will have to upload an image of their vaccination card to the company’s “FlyingTogether” website by Oct. 25 but is encouraging members to do it earlier. The union said failing to file the proof of vaccination could lead to disciplinary action.

“Many companies already require vaccinations and other job-related medical examinations,” the union said. “Medical staff, teachers and students, workers in the adult industry, and military service members are a few examples of occupations where vaccinations are required to protect employees from diseases.”

The union said United has not been explicit on what repercussions workers will face if they refuse to follow the new guidelines.

“However, it seems clear that vaccinations will soon be a requirement for anyone wishing to hold a position at United Airlines in the [United States],” the union said. “Vaccinations are now considered to be a safety requirement at the airline. Actions by employees that affect safety can result in discipline.”