Americans who arrived in California after being evacuated from China over the novel coronavirus this week will be quarantined, a rare action taken by the Centers for Disease Control and Prevention (CDC).
The Americans will be quarantined for a period lasting 14 days from when they left Wuhan, the Chinese city where the virus was first detected.
It's not clear whether any of the 195 individuals have the virus. Tests are ongoing, and Dr. Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Diseases, said a "negative result on a test will not help us confirm that people will be safe from this disease."
Officials also cited new research showing the virus could be spread even if an individual is not showing symptoms.
"While we realize this is an unprecedented action, this is an unprecedented threat," Messonnier said.
The Americans will be held at an air reserve base in Ontario, Calif., to avoid any risk of spreading the virus further.
The CDC has only confirmed six cases of the coronavirus in the U.S., and five of those people had recently traveled to China. The sixth is the spouse of one of the travelers.
"We would rather be remembered for overreacting than underreacting," Messonnier said.
The World Health Organization declared a global public health emergency Thursday as the number of cases of the disease nears 10,000.
Also on Thursday, the State Department officially warned Americans against travel to China amid the outbreak.