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Doctors in five U.S. cities will begin testing patients with flu-like symptoms for the new coronavirus, the Centers for Disease Control and Prevention said Friday.

The coronavirus test will only be given to patients who test negative for the flu. So far, the testing protocol will be implemented in Chicago, Los Angeles, New York, San Francisco and Seattle, though more cities will be added.

It's a sign that the U.S. is broadening its surveillance of the illness.

"This is just the starting point," Dr. Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Diseases, said during a press briefing Friday. "We plan to expand to more sites in the coming weeks."

Messonnier said the increased testing is part of an effort to determine whether the virus is spreading in communities across the U.S. "The results would be an early warning signal to trigger a change in our response strategy," she said.

Full coverage of the coronavirus outbreak

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So far, there is no indication that the coronavirus is spreading in this country or other countries outside of China.

As of Friday, just 15 patients had been diagnosed in the U.S., a small fraction of the nearly 64,000 cases reported in China, the majority of which were in the outbreak's epicenter, the city of Wuhan. All cases in the U.S. can be traced to travel from Wuhan.

Current restrictions on travel to and from China are meant to reduce the risk that the respiratory virus will gain a foothold in the U.S., but global travel constraints are unsustainable in the long term. And infectious disease experts know viruses historically do not respond to "no trespassing" signs.

For now, U.S. officials hope the outbreak can be controlled before restrictions are lifted.

Flu? Or coronavirus?

Symptoms of the coronavirus can mirror those usually associated with flu: fever, cough, body aches and extreme fatigue.

But at this point, unless a person has a direct link to Wuhan or a patient with the infection, experts say such symptoms are likely the result of the flu, rather than the coronavirus.

The CDC estimates that there have been at least 26 million cases of the flu in the U.S. this season. Roughly 14,000 people have died, including 92 children.

That means the flu continues to be a much bigger threat to American lives than the coronavirus.

More than 1,300 people have died in China from the coronavirus; there have not been any coronavirus deaths in the U.S.

There remain questions about how doctors in China are diagnosing new cases of the coronavirus. This week, physicians in Hubei province, where Wuhan is, changed their diagnostic criteria for the illness and are now identifying cases based on what the patients' lungs look like on CT scans, rather than waiting for lab results to confirm infection.

The change, meant to get care to patients faster, led to an apparent spike in reported cases.

However, there is no proven way for clinicians to distinguish whether lung damage seen on a CT scan is caused by coronavirus or other types of infection, such as the flu.

"We're seeking further clarity on how clinical diagnosis is being met to ensure other respiratory illnesses, including influenza are not getting mixed into the [coronavirus] data," WHO Director-General Dr. Tedros Adhanom Ghebreyesus said during a briefing Friday.

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