The World Health Organization said Saturday that it will be investigating reports from South Korea that the coronavirus has “reactivated” in 91 patients who were thought to have recovered to determine if there were testing errors or other anomalies. The WHO guidelines state that people are considered recovered from COVID-19 when they test negative twice within 24 hours, which usually takes place around two weeks after the onset of symptoms. South Korean health officials said Friday they are carrying out their own epidemiological investigations to determine the cause, which could be down to the patients not having fully recovered or taken extra time to shed the virus. “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,” the WHO said in a statement. “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.” It is currently thought that COVID-19 creates antibodies that protect anyone who has tested positive from contracting the virus again.