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As many as 10% of recovered coronavirus patients in China tested positive again after being discharged from the hospital, according to a report.

Doctors on the front lines of the outbreak in Wuhan, China — where the virus emerged — reported that between 3 and 10% of cured patients became reinfected with the illness, though it’s unclear whether they were contagious the second time, the South China Morning Post reported.

Tongji Hospital, which identified the first COVID-19 case, confirmed that five of 145 patients — a little over 3% — tested positive again in nucleic acid tests, according to state broadcaster CCTV.

“So far there is no evidence to suggest that they are infectious,” said Wang Wei, the hospital’s president.

He said the five patients who tested positive again did not have any symptoms and none of their close contacts had been infected.

Wang added that surveillance of similar patients showed that 80 to 90% had no trace of the virus in their system one month after being discharged from the hospital, according to the report.

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But, he said, these are just “small samples” and “not enough to assure us of the validity of our initial findings.”

“We need a large-scale epidemiological study to guide our disease surveillance and prevention works,” Wang said.

Meanwhile, other quarantine facilities in Wuhan have seen about 5 to 10% of their recovered patients test positive again. The findings were reported by the health news outlet Life Times, which is affiliated with state-run newspaper People’s Daily.

The studies come as health officials around the world are testing the concept of taking plasma from someone who has been infected, processing it and injecting the antibodies into a sick person to stimulate their immune system.