The agency now advises that vaccinated people be tested for the virus if they come into contact with someone with Covid-19, even if they have no symptoms. Previously, the health agency had said that fully vaccinated people did not need to be tested after exposure to the virus unless they were experiencing symptoms.

“Our updated guidance recommends vaccinated people get tested upon exposure regardless of symptoms,” Dr. Rochelle P. Walensky, the agency’s director, said in an email to The New York Times. “Testing is widely available.”

Fully vaccinated people should wear a mask in public indoor spaces after exposure, the agency said. Three to five days later, they should be tested for the virus.

If the results come back negative, they can stop wearing masks indoors. If results are positive, the infected should isolate at home for 10 days.