A medical staff member takes samples from a man at a walk-thru Covid-19 testing station set up at Jamsil Sports Complex in Seoul, South Korea, on April 3. Jung Yeon-je /AFP/Getty Images

A total of 141 people who had apparently recovered from Covid-19 have tested positive again, South Korea's Centers for Disease Control and Prevention (KCDC) said on Thursday.

KCDC deputy director Kwon Joon-wook said the agency did not know what caused the people to retest positive and was investigating.

Most experts think it's unlikely that somebody will be re-infected for the coronavirus soon after recovering. It's possible that issues with testing – or varying amounts of viral RNA in the body, which the tests look for – could explain why people tested positive after testing negative.

Kwon also said that the government is studying cultivated samples from the patients to determine whether the cases could be contagious. Kwon said the study will take about two weeks from today.

“Our KCDC workers are working day and night to collect samples and conduct studies,” Kwon said.

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