(Reuters) - American Airlines and United Airlines on Thursday will begin providing workers with Johnson & Johnson’s single-shot COVID-19 vaccine at Chicago’s O’Hare airport, according to letters sent to Chicago-based employees. So far, U.S. airlines are not requiring that employees take the vaccine, which the industry widely views as a key element to a recovery in travel, but they strongly encourage them to do so. United Airlines Chief Executive Scott Kirby said in January that the airline may make the COVID-19 vaccine mandatory for workers, something a spokeswoman said on Wednesday the company is still considering once vaccines are more widely available. “A vaccine mandate only works if the vaccine is widely available to all of our U.S. employees, which we know is not yet the case,” she said. In Chicago, United is working in partnership with the city and local health department on vaccine distribution, which will be available on a reservation-only basis for employees who live or work in Chicago and are either flight crew or over 65. “Unfortunately, not having enough vaccines in circulation continues to be the biggest challenge,” United said in its letter seen by Reuters. It also has airport point of dispensing (POD) sites for employees in Denver and Honolulu. An American Airlines spokeswoman said O’Hare is the first airport where it is rolling out a limited supply of the vaccines, which will be administered by Premise Health. Delta Air Lines launched a vaccination site at its Atlanta hub in February for employees eligible under Georgia’s distribution phase, and its Delta Flight Museum is one of four mass COVID-19 vaccination sites in the state. “We’re looking for opportunities to replicate this in other states where possible,” a spokesman said. Southwest Airlines said it does not have vaccination clinics because it does not have vaccines available to provide workers through an on-site third party, but strongly encourages employees to receive it.