Hundreds of people who recently traveled from China back home to the Bay Area are quarantining themselves under the watch of local public health officials in an unprecedented national effort to slow and possibly stop the spread of the new coronavirus in the United States.

Authorities have asked about 6,700 people in California to isolate and monitor themselves for symptoms of COVID-19, the disease caused by the new virus, according to the California Department of Public Health. Those individuals have been advised by the Centers for Disease Control and Prevention to stay home from work or school, avoid large crowds and limit their social interactions for 14 days — the maximum incubation period of the new virus.

These travelers are in addition to the roughly 1,000 people who are under strict quarantine at air bases in California, including about 170 people recently evacuated from a cruise ship in Japan who are being held at Travis Air Force Base in Fairfield.

The people who are self-isolating are considered at “medium risk” of becoming sick, which is a bit of a misnomer — almost none of them are expected to actually be infected with the new virus. But out of an “abundance of caution,” they are being asked to separate themselves from others to prevent the kind of widespread outbreak that’s blown up in parts of China, said Dr. Matt Willis, the Marin County health officer.

“There’s a conservative public health approach to try and identify every person who may be carrying this virus, to ensure that they’re not out in the community,” Willis said. “A lot of public health experts are thinking this is likely to be a disease that we’ll see more frequently globally, because it’s virtually impossible to contain a virus like this on one continent. But these steps will effectively slow the spread, even if they don’t prevent it from ever occurring.”

So far more than 76,000 cases of COVID-19 have been reported worldwide, including more than 2,200 deaths. There have been only 16 confirmed cases in the United States, all involving people who had either traveled to China recently or contracted the virus from a spouse who had traveled there. But 338 cruise ship passengers are currently under quarantine — including 25 whose test results from Japan came back positive. The United States is retesting the passengers and has not officially declared that they are infected.

California has had nine confirmed patients, four of whom were treated in the Bay Area. One new case was announced Thursday in Humboldt County.

It is unclear whether California has the largest number of returning travelers from China who are self-isolating. A CDC spokesman did not respond to questions about how many residents of other states are isolating themselves after returning from China, or whether the federal agency is tracking those figures.

The people being asked to self-isolate now are travelers who returned to the United States from China on or after Feb. 2. The CDC is funneling these travelers to 11 airports, including San Francisco International Airport, and advising them about how to protect themselves and their communities. The CDC also is giving names of returning travelers to local public health departments for monitoring during the two-week self-quarantine.

Though the individuals are told to avoid crowds and keep their distance from others as best they can, they are not under a mandatory quarantine and are not forbidden from leaving their homes.

When Rich Ren arrived at SFO on Feb. 2 following a Lunar New Year visit with relatives in China’s Xinjiang region, federal officials asked him whether he had been in Wuhan — the Chinese city where the new coronavirus originated — in the past 14 days.

Ren had not been to Wuhan, which would have required him to undergo a mandatory quarantine under close supervision, nor did he have a fever, cough or any other symptoms of COVID-19. Ren, 30, decided to self-isolate out of an abundance of caution.

He spent most of the next two weeks inside his home in San Mateo, where he worked remotely, read books on physics for fun and walked around his backyard for fresh air. His wife went out to get groceries.

He was “sleeping in a different bedroom” from his wife for the first few days upon his return, said Ren, who works in business development for a Bay Area tech company. “For the first two or three days, I felt a little bit nervous.”

His concern abated when he didn’t show any symptoms. On Tuesday, he went back to work.

People who develop symptoms of illness are instructed to call their doctor and local public health department, which arranges for them to report to a local emergency room for appropriate care. If doctors there determine someone may have developed COVID-19, a sample is taken and sent to the CDC for testing. Depending on how sick people feel, they may then be sent back home to continue quarantining themselves, or hospitalized and treated in an isolation unit.

More Information TELL US YOUR STORY Are you isolating yourself at home because of recent travel in China or other possible exposure to the new coronavirus? We’d like to hear from you. If you would like to share your story, contact Erin Allday at eallday@sfchronicle.com.

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Bay Area public health officials said that self-quarantines can be a burden on individuals and their families. People may need help getting food and other groceries. Children who are asked to stay home from school may struggle to keep up with classwork. And not all adults can easily work from home or take two weeks of sick leave from a job that can’t be done remotely.

“We’ve had a very high level of cooperation from folks,” said Dr. Chris Farnitano, Contra Costa County’s health officer, adding that some people have expressed frustration with the isolation, “but they realize this is a serious issue and these measures are important.”

“We’ve had some issues of folks being concerned about the loss of work, where they have to stay home and they’re losing income,” he said. “It can be a hardship for certain individuals.”

Monitoring the cases is also a heavy lift for public health departments that often have small teams assigned to communicable disease tracking. Farnitano said Contra Costa County is tracking “several dozen” individuals and gets a handful of new names from the CDC every day.

In San Francisco, authorities have monitored “hundreds” of people for coronavirus, said Dr. Susan Philip, director of disease prevention and control at the San Francisco Department of Public Health. Some of those individuals have reported symptoms of illness, but the city hasn’t yet reported any COVID-19 cases in residents.

“The majority of people who feel ill, it’s not going to be COVID-19. There’s still flu, there’s still the common cold, there are coronaviruses that are not the new coronavius,” she said. “But you have to proceed cautiously for the possibility.”

The case monitoring itself isn’t out of the ordinary, Philip added — the Public Health Department tracks all kinds of communicable diseases such as measles, tuberculosis and meningitis. “This is what we do in public health,” she said, “but the scale is unprecedented.”

Mallory Moench, Erin Allday and Catherine Ho are San Francisco Chronicle staff writers. Email: mallory.moench@sfchronicle.com, eallday@sfchronicle.com, cho@sfchronicle.com Twitter: @ErinAllday, @Cat_Ho