United Airlines will give its flight attendants an incentive to get the COVID-19 vaccine, but it will not require the shots.

Flight attendants who choose to get vaccinated will get up to three extra vacation days in 2022, the union representing United’s flight attendants said. To get the maximum number of days, flight attendants must have received at least one dose by June 9.

The Association of Flight Attendants and United want to get flight attendants vaccinated “as quickly as possible,” both for the health benefits and to be prepared if countries start requiring flight crews to be vaccinated, the union said.

Chicago-based United reached a deal with its pilots union last week that provides additional pay for pilots who choose to get the vaccine. As of Friday, about 80% provided proof of vaccination, United said.

The airline’s CEO, Scott Kirby, said earlier this year he wanted to make the COVID-19 vaccine mandatory for United’s workforce, as long as other companies also made shots mandatory for employees.

While employers can require the COVID-19 vaccine, many are encouraging, but not requiring, workers to get vaccinated.

Delta Air Lines is requiring all new hires be vaccinated, unless they qualify for an accommodation. American Airlines does not require vaccines but offered employees an extra vacation day in 2022 and $50 in employee-reward program points as an incentive.

Southwest Airlines said it encourages employees to get the vaccine, but has not offered an incentive.