Dr. Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases, on Sunday refused to rule out isolating areas of the US where coronavirus outbreaks are particularly severe.

“You know, you don’t want to alarm people, but given the spread we’ve seen, you know, anything’s possible,” Fauci said on Fox News Sunday. “And that’s the reason why we’ve got to be prepared to take whatever action is appropriate to contain and mitigate the outbreak.”

Fauci appeared on at least two television news shows to issue his warning. He also offered advice for those most vulnerable to infection and complications, saying they should avoid cruises, flights and large gatherings.

So far, there are more than 450 coronavirus cases reported in 33 states.

“I don’t think it would be as draconian as nobody in or nobody out,” Fauci said. “But if we continue to get cases like this, particularly at the community level, there will be what we call ‘mitigation,’ where we have to essentially do social distancing, keep people out of crowded places, take a look at seriousness, do you really need to travel, and I think it’s particularly important among the most vulnerable.

Several countries, including Italy, China, and South Korea, have severely restricted movements in the wake of the outbreak.