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At least 51 patients diagnosed as having fully recovered from the coronavirus in South Korea have tested positive a second time after leaving quarantine, according to officials.

The patients from Daegu all tested positive in a “relatively short time” after they were given the all-clear from their initial infections, the Korea Centers for Disease Control and Prevention (KCDC) said, according to the government-funded Yonhap News Agency.

A specialist team has been sent to conduct an epidemiological investigation in the city, which is the epicenter of the epidemic in South Korea, the agency says.

For now, the KCDC’s director-general, Jeong Eun-kyeong, believes it is likely the infection was re-activated after remaining dormant in the patients, as opposed to them being reinfected, the report said.

Either way, it strengthens fears that the contagion remains a hidden danger even after it appears to have gone — with whistleblowing Chinese doctors previously warning it is even deadlier the second time.

Doctors on the front lines in Wuhan — where the virus first originated — previously said as many as 10% of those assumed to have recovered had tested positive again. At least one patient — Li Liang, 36 — reportedly died from the disease after previously getting the all-clear.

Paul Hunter, an infectious diseases professor at the University of East Anglia, told MailOnline that the cases were far more likely to be “reactivations” — or even just a sign of current testing being flawed.

“Personally, I think the most likely explanation is that the clearance samples were false negative,” Hunter told the site.