People who have recovered from coronavirus have tested positive again.

CNN reports that this has been the case for 141 survivors in South Korea. South Korea's Centers for Disease Control and Prevention’s deputy director Kwon Jun-wook announced that the agency has launched an investigation.

A majority of health experts believe that somebody can't be re-infected from COVID-19 again after recovering. Additionally, problems with testing could result in false positives.

Kwon shared that the government is analyzing cultivated samples from patients to figure out if the cases are contagious, and if those patients have an antibody that signals they’ve recovered. The study’s results will arrive two weeks from Friday. “Our KCDC workers are working day and night to collect samples and conduct studies,” he said.

The same thing has happened in China. Around 5 to 10 percent of recovered patients in Wuhan, China tested positive for the virus again at the end of March.

The U.S. CDC has said the immune response to COVID-19 and the extent of immunity is unknown. Three antibody tests have been granted emergency approval by the FDA, which assess recovery from the virus. However, the tests’ accuracy has come into question.

South Korean health authorities have said it’s very possible the virus was reactivated—rather than the people becoming reinfected—since they were tested soon after being freed from quarantine. They added that the virus may remain dormant in the body and once again attack respiratory organs when reactivated.