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The agency has not yet been able to trace the source of the infection.

A California coronavirus patient is suspected of being the first person to be infected through community exposure in the United States—they went untested for four days because of restrictive federal rules.

The patient arrived at UC Davis Medical Center in Sacramento County on Feb. 19, but was not tested for the coronavirus (also known as COVID-19) until Feb. 23, the medical center said in a statement.

“Upon admission, our team asked public health officials if this case could be COVID-19. We requested COVID-19 testing by the [Centers for Disease Control and Prevention], since neither Sacramento County nor the California Department of Public Health is doing testing for coronavirus at this time. Since the patient did not fit the existing CDC criteria for COVID-19, a test was not immediately administered. UC Davis Health does not control the testing process,” the medical center said.

The CDC has restricted coronavirus testing to patients who either recently traveled to China or who knew they had contact with an infected individual. The patient, who was initially suspected of having a viral infection, had no history of travel to China or other areas where the virus is widespread and had no known link to a returning traveler. 

The patient was initially hospitalized at another Northern California hospital and was already on a ventilator upon being transferred to UC Davis. After four days at the medical center, the CDC finally ordered a coronavirus test on Sunday, which came back positive on Wednesday, the medical center said in its statement.

The CDC said it was possible the patient had been exposed to an infected returning traveler, but the agency has not yet been able to trace the source of the infection.

“It’s possible this could be an instance of community spread of COVID-19, which would be the first time this has happened in the United States,” the agency said in its statement. “Community spread means spread of an illness for which the source of infection is unknown.”

The coronavirus outbreak has now sickened 82,400 people in 47 countries and left at least 2,804 people dead, all but 60 in mainland China. Sixty Americans have now tested positive for the virus, the majority of whom were aboard the Diamond Princess cruise ship that was quarantined in Japan.

President Trump on Wednesday announced that Vice President Mike Pence would spearhead the government’s response to coronavirus. While Pence will oversee the new coronavirus task force, he on Thursday tapped longtime health official and ambassador-at-large Debbie Birx to serve as the White House coronavirus response coordinator. Birx has worked in the medical field for decades, mostly focusing on preventing the spread of HIV/AIDS.

While the government steps up its response behind the scenes, the president continues to publicly downplay the danger of a possible widespread outbreak.

“The risk to the American people remains very low,” Trump said. “We have the greatest experts, really in the world, right here.

Trump’s reassurances meant little to the stock market, which continues to fall amidst fears of a possible pandemic. The Dow Jones, Nasdaq, and S&P 500 have all dropped by about 10% since last Friday and are on pace for their worst week since the 2008 financial crisis.

The president’s statements are also in stark contrast to the CDC’s, which on Wednesday warned Americans that the disease is all but certain to spread throughout the United States.

“It’s not so much of a question of if this will happen anymore but rather more of a question of exactly when this will happen and how many people in this country will have severe illness,” Dr. Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases, said in a briefing. “We are asking the American public to prepare for the expectation that this might be bad.”

If the patient in Northern California did indeed contract coronavirus from community exposure, then it could be a sign that many more cases are likely to be diagnosed in the U.S. in the coming days, according to Jennifer Nuzzo, epidemiologist and senior scholar at the Johns Hopkins Center for Health Security. 

“It would confirm what we have long suspected — that there is a good chance there already are people infected in this country and that the virus is circulating undetected,” Nuzzo told the Washington Post. “It points to the need for expanded surveillance so we know how many more are out there and how to respond.”