Eight United Airlines employees based at Dallas-Fort Worth International Airport are suing the carrier over its coronavirus vaccine mandate program, the most aggressive in the aviation industry.

The employees filed the lawsuit in a North Texas federal court Wednesday, saying that the Chicago-based airline is not taking into account their religious beliefs because CEO Scott Kirby said few personal or religious exemptions would be granted.

A week ago, United told employees that those who claim religious exemptions would be placed on unpaid medical leave.

The lawsuit said employee requests for religious exemptions were met with “a draconian threat of years of unpaid leave” and amounted to an adverse employment action intended to force them to “forgo their religious beliefs and receive the COVID-19 vaccine.”

“United’s actions have left [the employees] with the impossible choice of either taking the COVID-19 vaccine, at the expense of their religious beliefs and their health, or losing their livelihoods,” the lawsuit said.

The employees are represented by Dallas law firm Stewart, Wiegand and Owens.

The lawsuit is the latest in a string of legal maneuvers across the country as both corporate and government entities step up pressure on the public to get vaccines for the COVID-19 virus with the delta variant still raging across the country.

It also comes shortly after a White House mandate that all companies with more than 100 employees must require workers to be vaccinated.

United is reviewing the lawsuit, “but at this point, we think it’s without merit,” a spokesperson for the company said in a statement.

“The most effective thing we can do as an airline to protect the health and safety of all our employees is to require the vaccine — excluding the small number of people who have sought an exemption, more than 97% of our U.S. employees are vaccinated,” the United spokesperson said in a statement. “And we’ve been encouraged by the overwhelmingly positive response from employees across all work groups since we announced the policy last month.”

United’s mandate applies to all 67,000 of its U.S. workers. United announced Wednesday that 97% of its employees are vaccinated, compared to 90% that were vaccinated before its mandate.

The airline industry has been uniquely challenged by the virus. U.S. airlines lost $35 billion last year due to the downturn even after receiving billions in aid and loans from the federal government.

Passenger traffic is still down 20% from pre-COVID-19 levels, and airlines have reported another slowdown in ticket sales with the latest virus surge. And mask mandates, required first by airlines, then the federal government to fly, have resulted in unruly and sometimes violent passenger interactions with flight crews.

Airlines have all taken a pro-vaccine stance but have wavered in how far they want to go. Dallas-based Southwest Airlines and Fort Worth-based American Airlines are not requiring vaccines for employees yet but gave extra pay or extra vacation days to people who do get vaccinated. Those airlines also are requiring unvaccinated workers who get sick or are exposed to the virus to use their own vacation and sick time.

Atlanta-based Delta Air Lines is making employees get vaccinated or submit to regular COVID-19 testing to stay on the job.

Most airlines also are requiring new employees to be vaccinated.

The lawsuit against United claims that the employees — including pilots, flight attendants, maintenance workers and customer service workers — are claiming religious and medical reasons for not getting the vaccine. Several have said their requests for exemptions were already denied.

Several of the employees told the airline they can’t get inoculated because tissue grown from aborted fetal stem cell lines was used to develop and test all of the vaccines approved by federal medical authorities. Medical and religious authorities, however, have argued that hundreds of modern drugs and vaccines, including the chickenpox vaccines and Tylenol, have similarly used the legacy aborted stem cell lines, which were harvested in the 1980s and grown in labs over the past four decades.

Kirby, the former president of American Airlines and owner of a home in North Texas, said on CBS’ Face The Nation on Sunday that the airline would be ready if the federal government required all passengers to be vaccinated as well.