About 51 patients classed as having been cured in South Korea have tested positive again, the CDC said in a briefing on Monday. Rather than being infected again, the virus may have been reactivated in these people, given they tested positive again shortly after being released from quarantine, said Jeong Eun-kyeong, director-general of the Korean CDC.

“While we are putting more weight on reactivation as the possible cause, we are conducting a comprehensive study on this,” Jeong said. “There have been many cases when a patient during treatment will test negative one day and positive another.”

A patient is deemed fully recovered when two tests conducted with a 24-hour interval show negative results.

The Korean CDC will conduct an epidemiological probe into the cases, Jeong said.

South Korea was one of the earliest countries to see a large-scale coronavirus outbreak, but the country has seen just 200 deaths and a falling new case tally since peaking at 1,189 on Feb. 29. One of the world’s most expansive testing programs and a tech-driven approach to tracing infections has seen Korea contain its epidemic without lockdowns or shuttering businesses.

Fear of re-infection in recovered patients is also growing in China, where the virus first emerged last December, after reports that some tested positive again — and even died from the disease — after supposedly recovering and leaving hospital. There’s little understanding of why this happens, although some believe that the problem may lie in inconsistencies in test results.

As of Wednesday, South Korea had 10,384 virus cases, with 6,776 released from hospital, according to data compiled by Johns Hopkins University and Bloomberg News.

Epidemiologists around the world are in a race to find out more about the virus that causes Covid-19. The pathogen’s rapid global spread has recently seen the focus shift to patients who contract the virus but display few or atypical symptoms. Korea has been at the forefront of tracking these cases, which are causing particular concern in China, where the epidemic is showing signs of coming under control.

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