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.