Public health authorities plan to start widespread surveillance for the new coronavirus in five U.S. cities, including San Francisco, by tapping into an existing network of labs that already test for influenza and other common viruses.

The Centers for Disease Control and Prevention did not say when the labs would begin testing for the new virus, which causes a respiratory illness called COVID-19 that has sickened more than 67,000 people worldwide. So far, 15 cases have been reported in the United States, including eight in California.

The CDC said Friday that the risk of infection from the new virus remains low for the general public. But infectious disease experts said it makes sense to begin more large-scale surveillance for the virus soon, both to track down individual cases that may be missed by other screening efforts, and to identify potential clusters of cases right away.

“It’s a perfectly rational, inexpensive, quick way to see if, first, we have a problem now — and most people assume we do not,” said Dr. Art Reingold, an infectious disease expert at UC Berkeley. “And second, if we do start having a problem, we’ll pick it up faster this way.”

In a news conference Friday, CDC officials said testing for coronavirus would start in San Francisco, Los Angeles, Seattle, Chicago and New York City, using state- or city-run laboratories that are part of the national influenza surveillance network. Results from that network are published weekly in the CDC’s FluView report.

The CDC did not say which parts of the influenza network would be involved. The surveillance network collects and analyzes samples from a variety of sources, including hospitals and outpatient clinics.

At facilities in the network, staff collect oral or nasal samples from patients who report flu-like symptoms, including fever, cough, sore throat, body aches and fatigue. Those samples are sent to labs that test for influenza and other viruses; some labs further test for specific strains of influenza.

The results are forwarded to the CDC, which uses the information to help mount a public health response to seasonal flu. A few thousand samples are tested each week at the state and local public health labs; it wasn’t clear Friday how many samples come from San Francisco.

Once coronavirus is added to the surveillance, samples will only be tested for the new virus if they are negative for other more common viruses, said Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the CDC.

“This is leveraging our existing influenza surveillance. It will help us detect if and when this (coronavirus) is spreading in the community,” Messonnier said. “Results from this surveillance would be an early warning signal to change our strategy.”

She added that other locations would be added to the surveillance network. “This is just a starting point,” she said.

Reingold said the CDC likely picked San Francisco and the other four cities to start testing because they have the most direct contact with China, the epicenter of the outbreak. Of the 15 cases reported in the United States, at least six are patients who live in or near one of the five cities.

The United States so far has been hunting for COVID-19 cases by screening incoming travelers from China, including placing several hundred evacuees from the center of the outbreak in quarantine. Every case so far identified in the United States can be traced directly to an exposure to the virus in China.

Suspected cases currently are confirmed by testing at the CDC’s main labs in Atlanta. States were supposed to begin conducting their own tests this week, but a flaw in the testing kits made many of them unusable. The CDC is replacing the kits but has not yet said when new tests will be sent.

Community surveillance testing will be separate from diagnostic testing, which is only done on people who have symptoms such as fever and cough who have recently traveled to China or been in close contact with someone who has already been diagnosed with COVID-19. The CDC has tested 443 individuals so far.

Lab testing of the broader community will help reassure public health officials that they are keeping the virus at bay, infectious disease experts said.

But perhaps more important is to have a robust surveillance system in place to identify small outbreaks in the United States if the virus does manage to spread here, said Dr. Charles Chiu, an infectious disease expert who heads the viral diagnostics laboratory at UCSF.

“With only 15 (coronavirus) cases so far, we haven’t had a need for this type of active surveillance yet,” Chiu said. “But it perhaps is warranted given the number of cases we’ve seen in China and the possibility we may see many more cases in the United States.”

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com