Survivors of COVID-19 who re-tested positive for the disease after making a full recovery were no longer infectious or a risk to their loved ones, South Korea's health agency announced Monday, adding a new clue to the ongoing mysteries surrounding Covid-19 immunity.

On Monday, the country's Centre for Disease Control (KCDC) announced that it had studied 400 recovered COVID-19 patients, 285 of whom re-tested positive for the virus after they recovered; the KCDC then traced the close contacts of those individuals and found zero new cases of infection among 790 close contacts, Forbes reported.

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The KCDC concluded that COVID-19 survivors therefore do not need to be quarantined for two weeks after hospital release as they are not infectious after recovery, according to CNN.

The recovered patients retested positive because the test, known as a diagnostic polymerase chain reaction (PCR) test, falsely identified dead viral matter as active COVID-19 infection, according to health officials.

It is unclear whether the country and test manufacturers are working to remedy, or have fixed, this testing flaw.

South Korea's findings could represent an important development in determining survivor immunity, which the U.S. Centers for Disease Control and Prevention says is still "not yet understood."