The United States government has imposed a federal quarantine on 195 people who were evacuated on Wednesday from Wuhan, China, to a California military base, officials said on Friday.
The action means that the group will be held at March Air Reserve Base in Riverside, Calif., for 14 days, to make sure that they are not infected with the Wuhan coronavirus that has sickened more than 9,800 people in China and killed more than 200 people.
Since their arrival, members of the group have had their noses and throats swabbed to test for the virus, and their temperatures taken several times a day. They were originally told that they would be detained for at least 72 hours, and possibly 14 days. Two weeks is thought to be the upper limit of the incubation period; if symptoms don’t start by then, a person can be cleared as not infected.
Until Friday, the detention was unofficial and the group stayed at the base voluntarily, except for one member, who tried to leave on Wednesday and was quarantined by Riverside County.
Federal law authorizes the C.D.C. to impose quarantines, but the power is rarely used. The last large-scale quarantine was imposed during the 1918-1919 Spanish flu pandemic.
So far, six people in the United States are known to be infected with the virus. Five had been to Wuhan and the sixth is the husband of one of those patients. He had not traveled to Wuhan, but contracted the illness from his wife after she returned home to Chicago. The other cases have been in Washington State, California and Arizona.
More than 100 people in the United States are being monitored as “patients under investigation,” because they have symptoms like fever and cough and have been to Wuhan or in contact with patients.
Hundreds of other contacts of patients have been told to reach out to health officials immediately if they become ill.
Concern about spread of the virus has increased as reports have emerged suggesting that people who are infected may be contagious even before symptoms develop.
German physicians who identified the first coronavirus case in their country have reported that the patient was infected by a colleague visiting from China who was not yet sick herself.
That the disease may be transmitted before infected people show symptoms will make the disease harder to contain, experts say.
A letter published in the New England Journal of Medicine said the German patient was a healthy 33-year-old businessman who developed a sore throat, chills and muscle aches on Jan. 24, with a fever spiking to 102.4 the next day, along with a cough. He felt better a day later and returned to work on Jan. 27.
Before he became ill, he had met with a Chinese business partner from his company near Munich, on Jan. 20 and 21. She was a resident of Shanghai who had been in Germany from Jan. 19-22. Though she was healthy during the visit, she got sick during her flight back to China, where she tested positive for the new coronavirus on Jan. 26. She informed her company, which traced her contacts to alert anyone she might have exposed.
The 33-year-old businessman was contacted, and though he was already feeling better, he tested positive for the virus.
On Jan. 28, three more employees of the company tested positive for the coronavirus, only one of whom had contact with the woman from Shanghai. The others were apparently infected by their male colleague, before he developed symptoms. All of them were admitted to the hospital, where they were isolated and monitored, and none has developed severe illness.
The authors of the letter in the New England Journal of Medicine, who are from Munich and Berlin, said it was “notable that the infection appears to have been transmitted during the incubation period of the index patient, in whom the illness was brief and non-specific.”
“It does mean that transmission is more readily achieved than it was in SARS,” an earlier coronavirus disease, said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center. “The report from Germany indicates quite conclusively that spread during the incubation period when the person is not sick themselves can take place.”
But, he said, “We don’t know how frequently it happens.”
“This is a kind of natural experiment, where people were nicely shown to be in a chain of transmission, and it sounds very, very plausible,” Dr. Schaffner said.