NEW YORK: From Wall Street to Silicon Valley, slaughterhouses to airlines, more American firms are demanding their employees get Covid-19 vaccines to return to the workplace, and are willing to fire them if they violate the rule.

CNN news network terminated three employees who flouted the policy and went to work without being immunized against the coronavirus.

“Let me be clear — we have a zero-tolerance policy on this,” CNN Chair Jeff Zucker wrote in a memo to employees that was tweeted Thursday by a reporter for the channel.

While experts say they have the legal authority to do so, corporations had been reluctant to take this step earlier in the pandemic, possibly fearing a political backlash.

But as firms have moved to re-open their offices, US vaccination rates have stalled at around 50 percent and infections have surged due to the Delta variant.

United Airlines boss Scott Kirby, who in January said he was considering a vaccine requirement, on Friday announced 67,000 US-based workers will have to be vaccinated by the end of October.

“We know some of you will disagree with this decision,” he wrote in a message to employees.

“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.”

Employers have the right to require workers returning to their offices to be vaccinated, with exemptions for medical or religious reasons, according to the US Equal Employment Opportunity Commission (EEOC).

Eric Feldman, professor of health law and medical ethics at the University of Pennsylvania, said CNN was justified in firing workers, if the rules were clearly laid out.