Three US citizens evacuated from Wuhan, China, tested positive for the novel coronavirus this week.

All three were under a mandatory 14-day federal quarantine at US military bases – two in California and one in Texas. It’s the first time in more than 50 years a quarantine order has been issued. But these probably won’t be the US’ last coronavirus patients, the Centers for Disease Control and Prevention (CDC) said on Thursday.

“There will likely be additional cases in the coming days and weeks, including among other people recently returned from Wuhan,” the CDC said in a statement.

In anticipation of additional cases, the US government has established quarantine centers at 15 military facilities across the country.

“Now is the time to act so that we can slow the introduction and impact of this virus in the US,” Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, said February 5. The last time the US enforced large-scale quarantines was during the Spanish flu pandemic of 1918.

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Foto: A charter plane lands at March Air Reserve Base in Riverside, California, on January 29, 2020, with passengers evacuated from Wuhan.sourceMatt Hartman/AFP/Getty Images

In total, the novel coronavirus outbreak has killed at least 1,380 people and infected more than 64,000. It has spread to 25 countries beyond China, with 15 cases in the US. The latest was confirmed Thursday at the Lackland Air Force Base near San Antonio, Texas.

15 military bases across the US can hold quarantined people

Around 800 US citizens have been evacuated from Wuhan in the last month. Five primary military bases – three in California, one each in Colorado, Nebraska, and Texas – can hold up to 1,000 quarantined people. The additional locations, which were announced February 6, can house up to 20 people, but none have been used yet.

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On Tuesday, the first 195 evacuees were permitted to leave the March Air Reserve Base in Riverside, California. It marked the first successful US quarantine in nearly 60 years.

“They have been watched more closely than anyone else in the United States at this point in time,” Nancy Knight, director of the Division of Global Health Protection at the CDC, said in a briefing.

Jamie Fouss, the US consul general in Wuhan, was among those quarantined in Riverside. He said evacuees tried to live normal lives during their two-week isolation – they played trivia games, took Zumba and art classes, and sent their children to school.

None of the passengers tested positive for the virus.

About 600 additional quarantined US citizens are still waiting for the all-clear to go home, though. At the Marine Corps Air Station Miramar, 322 people petitioned for more stringent virus testing after after one evacuee who’d tested positive for the coronavirus was accidentally sent back to the base from the UC San Diego Medical Center.

March Field Air Base

Foto: The entrance to March Air Reserve Base in Riverside, California, on January 29, 2020.sourceMatt Hartman/AFP/Getty Images

The Department of Health and Human Services is responsible for carrying out federal quarantines – the DOD is not involved in treating or observing the quarantined people.

The three quarantined people who got the coronavirus – one at the Lackland Air Force Base and two at the Marine Corps Air Station Miramar – were moved to local civilian hospitals.

Choosing a quarantine length

The incubation period for the novel coronavirus – the time between exposure and the onset of symptoms – is believed to be between two and 14 days, according to the CDC.

The US and many other countries formulated their quarantine rules based on that 14-day maximum.

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Foto: A medical worker wears a mask as buses carrying British nationals arrive at the Arrowe Park Hospital in Merseyside for a 14-day quarantine period.sourcePeter Byrne/PA Images/Getty Images

“It’s widely accepted that there’s a 14-day rule of thumb,” Stephen Morse, an epidemiologist at Columbia University, previously told Business Insider. “That’s how long you have to wait to go back to your daily life.”

However, new research from a group of Chinese scientists suggested the virus’ incubation period could be as long as 24 days. If true, that could suggest the current quarantine periods are not sufficient. But the study has yet to be peer reviewed.

“The incubation period is obviously really important for us as we look to make sure that we’re releasing these people safely from quarantine,” Messonnier said on Wednesday. But she added, “We still think that for today, for now, 14 days is the right interval to use.”

Aria Bendix contributed reporting to this story.