South Korean officials on Friday reported 91 patients thought cleared of the new coronavirus had tested positive again.

Jeong Eun-kyeong, director of the Korea Centers for Disease Control and Prevention (KCDC), told a briefing that the virus may have been “reactivated” rather than the patients being re-infected.

South Korean health officials said it remains unclear what is behind the trend, with epidemiological investigations still under way.

The prospect of people being re-infected with the virus is of international concern, as many countries are hoping that infected populations will develop sufficient immunity to prevent a resurgence of the pandemic.