Formal U.S approval of the Pfizer Inc /BioNTech SE COVID-19 vaccine will make it nearly impossible to successfully challenge mandates by employers, legal experts said.

The decision by the Food and Drug Administration to give full approval to the vaccine is “seismic,” said Brian Dean Abramson, an author on vaccine law.

He said it will become extremely difficult to challenge the FDA’s decision and the mandates that flow from it.

On Monday, the Pfizer/BioNTech vaccine became the first to secure full FDA validation, prompting calls for governments and private employers to make the shots mandatory.

COVID-19 vaccines have been available in the United States since December under the an emergency use authorization (EUA) by the FDA.

Language in the EUA law states that recipients must be informed of benefits and risks of the vaccine and given the option to accept or refuse it.

That language raised some uncertainty regarding employer mandates, which are usually considered legal, said Dorit Reiss, a professor at UC Hastings Law. “With full approval, that is removed.”

Following the FDA announcement on Monday, CVS Health Corp , Chevron Corp. – the second-largest U.S. oil producer – and Goldman Sachs issued mandates for some employees.

Legal experts said there already was a growing consensus that employers could mandate an emergency vaccine. During the pandemic, both the Equal Employment Opportunity Commission and the Department of Justice issued guidance in support of vaccine mandates, provided exceptions were made for medical conditions and religious beliefs.

But emergency vaccine requirements have not gone unchallenged. At least a dozen lawsuits have been filed, mostly by students against colleges, but also by employees fighting allegations of wrongful termination for refusing a shot.

Most of the cases prominently feature arguments that vaccines approved on an emergency basis could not be required, seizing on language in the emergency authorization law that requires consent from the vaccine recipient.

Isaac Legaretta, for example, said he was never told he could refuse the vaccine required by the Dona Ana Detention Center in New Mexico where he worked.

“Quite the opposite, he was advised that he would be fired if he did so,” said the Legaretta lawsuit, which was filed in February and is pending.

In July, in one of the few rulings involving a private employer, a federal judge in Texas upheld vaccine mandates for employees at a Houston Methodist Hospital, finding the employees misunderstood the language in the EUA law.

Legal experts said challenges to vaccine mandates will almost certainly persist, particularly against public employers or public universities and colleges, which involve allegations of governments infringing on an individual’s Constitutional rights. That argument does not apply to private employers.

But as long as the government is requiring the vaccine as a condition of employment or education, legal experts said those will be difficult cases to make.

“You can always go work for somebody else or go to a school that doesn’t require a vaccine,” said Jeffrey Nolan, an attorney with Holland & Knight, which represents employers.

Many employers have tried to use incentives such as gift cards and time off to encourage vaccinations. That approach seems to have run its course, legal experts said.

With full FDA approval, employers appear ready to move toward ordering staff to get vaccinated.

Samantha Monsees, an attorney with Fisher Phillips, which represents employers, said: “I think based on my workload in the last two days the FDA approval is going to tip the scales with a lot of employers.”