The World Health Organization said it is investigating reports out of South Korea that some patients who had recovered from the coronavirus tested positive again after initially testing negative for COVID-19. On Friday, South Korea officials said 91 patients who were thought to have recovered from the coronavirus tested positive again.

Health officials in South Korea are speculating that these may be cases in which the virus was reactivated rather than people having been infected again. “While we are putting more weight on reactivation as the possible cause, we are conducting a comprehensive study on this,” said Jeong Eun-kyeong, director-general of Korea’s Centers for Disease Control and Prevention. “There have been many cases when a patient during treatment will test negative one day and positive another.”

The WHO said it was aware of the reports and wanted more information to try to figure out what they mean. “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 said in a statement. “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.”

According to the WHO guidelines, a COVID-19 patient can be discharged from the hospital after testing negative for the coronavirus in two separate tests given at least 24 hours apart. Some experts say that the key may be whether the patients have symptoms. “If you don’t have symptoms but have a positive test, it may be that you have dead virus that’s still being picked up, but you can’t transmit,” ABC medical contributor and infectious diseases physician Todd Ellerin said.

Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases, said Friday that the government is looking into whether it would be a good idea to start granting certificates of immunity from the coronavirus. “I mean, it’s one of those things that we talk about when we want to make sure that we know who the vulnerable people are and not,” he said. “This is something that’s being discussed. I think it might actually have some merit, under certain circumstances.” Some experts warn though that it’s too early to be talking about immunity from the coronavirus. Although recovered COVID-19 patients appear to have antibodies for at least two weeks, it’s still not clear whether that would continue to be true in the future. “We simply don’t know yet what it takes to be effectively protected from this infection,” Dawn Bowdish, a professor of pathology and molecular medicine and Canada Research Chair in Aging and Immunity at McMaster University in Ontario, tells Scientific American.