CNN fires three employees who went into the office unvaccinated.

CNN said on Thursday that it had fired three employees who violated its coronavirus safety protocols by going to the office unvaccinated, one of the first known examples of a major American corporation’s terminating workers for ignoring a workplace vaccination mandate.

CNN had said this year that employees would need to be vaccinated against the virus in order to return to its American offices. The network has been relying on an honor system rather than requiring proof of vaccination status.

“In the past week, we have been made aware of three employees who were coming to the office unvaccinated. All three have been terminated,” CNN’s president, Jeff Zucker, wrote in an internal memo on Thursday that The New York Times obtained. “Let me be clear — we have a zero-tolerance policy on this.”

Mr. Zucker did not say where the employees worked, what positions they held or how their violations of the vaccine mandate were discovered. CNN declined to comment beyond his memo.

“We expect that in the weeks ahead, showing proof of vaccination may become a formal part” of the process for gaining access to the network’s workplaces, Mr. Zucker added.

Many large companies — including Facebook and Google — have said they will require employees to be vaccinated against the virus in order to return to the office. Some companies, including The Washington Post, have gone further by making vaccination a condition of further employment.

But few corporations have described exactly what kind of consequences would arise for employees who violated the vaccine requirements.

In his memo on Thursday, Mr. Zucker announced that CNN had postponed its formal Sept. 7 return to the office for American employees. CNN has not determined a new date, but Mr. Zucker said sometime in October “seems reasonable at this point.” He wrote that “a little more than a third” of CNN’s American newsroom employees were now regularly working in the office again.