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WASHINGTON (NEXSTAR) – New research shows COVID-19 and its symptoms can last much longer than the 10-day benchmark established by the Centers for Disease Control and Prevention for Americans to return to work or school following onset of the virus.

Research published Tuesday in the British Medical Journal suggests patients may need to wait longer than a month to really know whether they’ve been cleared of the virus.

Once cleared, researchers found many patients battle symptoms that can last months. Most commonly, those symptoms range from aches and pains to loss of smell to a cloudy head.

“We think that this long-term damage may in part be due to vascular damage, kind of a footprint that the virus leaves even when it’s gone from the body,” Dr. William Li said in an interview with CNN.

Li also noted in the interview it’s difficult to determine when someone is “clear” of the virus because so much remains unknown about COVID-19.

The research also found one in five tests comes back as a false negative. This suggests some patients who thought they were clear of the virus could be spreading it without knowing it.

As of Wednesday morning, more than 6 million coronavirus cases had been confirmed in the United States, according to Johns Hopkins University. Global cases have now exceeded 25 million.