CNN has fired three staff members for working in the office despite being unvaccinated against the coronavirus, in an incident that highlights the potential challenges facing employers who mandate inoculations amid a surge of the highly transmissible delta variant in the United States.

Jeff Zucker, the cable network’s president, wrote in a Thursday memo obtained by The Washington Post that CNN was “made aware” in the past week of three employees violating its policy that only fully inoculated people work from its buildings.

“All three have been terminated. Let me be clear. We have a zero-tolerance policy on this,” Zucker wrote. He added that staffers working with colleagues in the field must also be inoculated, even if they do not enter an office owned by the network.

CNN had been using an honor system to monitor its employees’ vaccination status, although Zucker said that might change in the coming weeks. Zucker’s note did not indicate how the company learned of the vaccination statuses of the fired employees, whom it did not name, or include further details.

The WarnerMedia-operated network is one of the highest-profile employers in the United States to have fired workers who violate its vaccine mandate. Many people who resigned or were terminated as a result of vaccine mandates had worked in the public or health-care sectors.

Last month, New Jersey’s largest health-care system said that it had fired six senior medical workers who refused to get vaccinated. In March, a deputy sheriff in Durham County, N.C., was allegedly terminated for refusing to disclose his vaccination status.

And in June, the Houston Methodist hospital system fired or accepted the resignations of more than 150 employees who did not comply with its inoculation mandate. The move followed the ruling by a federal judge in Texas dismissing a lawsuit against Houston Methodist over its vaccination requirement.

Such terminations could become more common as employers nationwide — including Google, Facebook, Uber, Morgan Stanley and Tyson Foods — increasingly make inoculations mandatory for many employees amid a resurgence of the coronavirus in the United States. The Washington Post has also made inoculation a condition of employment.

Last week, President Biden ordered all federal government workers to receive shots or wear masks and undergo repeated tests for the virus.

On Thursday, Los Angeles County’s court system also told its 5,000 workers to provide proof of inoculation within 45 days of the Food and Drug Administration finalizing approval of coronavirus vaccines, the Los Angeles Times reported.

The push to make inoculation a condition for employment has been met with some pushback from civil libertarians and people skeptical about the safety of coronavirus vaccines, although all vaccines authorized in the United States have undergone vigorous testing.

The Justice Department issued guidance last month that existing law doesn’t prohibit workplaces from mandating vaccine shots. Lynn N. Hughes, the federal judge who ruled in favor of Houston Methodist, also wrote in his decision that Texas law only protects employees from termination when they are asked to commit an illegal act carrying criminal penalties.

Jeremy Barr contributed to this report.