Americans, especially those who are vulnerable, may need to stop attending big gatherings as the coronavirus spreads through US communities, a top health official said on Sunday, adding that the possibility of large-scale quarantines cannot be ruled out.

Anthony Fauci, the head of the infectious diseases unit at the National Institutes of Health, said on NBC’s “Meet the Press” that after initial missteps distributing tests, there should be 400,000 more tests available by Monday and 4 million by the end of the week.

In the US, 19 people have died out of around 450 reported cases of coronavirus, which originated in China last year and causes the sometimes deadly respiratory illness COVID-19. The outbreak has killed more than 3,600 globally.

More than half of the 50 US states have reported cases. As the outbreak spreads, daily life has been increasingly disrupted, with concerts and conferences cancelled and universities telling students to stay home and take classes online.

To contain the outbreak in China, the government quarantined millions of people for weeks. Italy has announced similar measures, locking down 16 million people in the north of the country.

Asked about those quarantines, Fauci said he could not rule out similar actions in the US. “You don’t want to alarm people, but given the spread we see, anything is possible, and that’s the reason why we’ve got to be prepared to take whatever action is appropriate to contain and mitigate the outbreak,” he said on “Fox News Sunday”. He urged those most at risk from coronavirus to limit travel.

“If you’re a person with an underlying condition and you are particularly an elderly person with an underlying condition you need to think twice about getting on a plane, on a long trip, and not only think twice, just don’t get on a cruise ship,” Fauci said on “Meet the Press”.