The World Health Organization (WHO) confirmed on Saturday it is investigating reports of some recovered coronavirus patients testing positive for the illness after initially testing negative.

The move is in response to a report from South Korea on Friday that 91 patients who had been cleared of COVID-19 and were being prepared for discharge tested positive again. Officials say that, rather than being reinfected, patients may be suffering from a “reactivated” coronavirus.

“We are aware of these reports of individuals who have tested negative for COVID-19 using PCR (polymerase chain reaction) testing and then after some days testing positive again,” the WHO told The Hill in a statement.

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“We are closely liaising with our clinical experts and working hard to get more information on those individual cases. It is important to make sure that when samples are collected for testing on suspected patients, procedures are followed properly,” it added.

The WHO’s clinical management guidelines state that patients can be discharged from the hospital after two negative coronavirus test results at least 24 hours apart.

Current studies have shown that as many as 14 days can take place between the start of symptoms and clinical recovery in mild cases of the coronavirus.

“We are aware that some patients are PCR positive after they clinically recover, but we need systematic collection of samples from recovered patients to better understand how long they shed live virus,” the WHO said.

The novel coronavirus has infected more than 1.7 million people worldwide and killed more than 103,000.