South Korean officials have announced 91 patients who were thought to have recovered from COVID-19 have re-tested positive.
Jeong Eun-kyeong, the director of the Korea Centers for Disease Control and Prevention (KCDC), says the virus appears to have been "reactivated" rather than the patients being reinfected, Reuters reported.
The number of patients has grown after 51 were announced on Monday and officials say it brings doubt into what it means for a patient to be cleared of the virus and when they should be allowed to return home.
Bloomberg reported a patient is currently deemed fully recovered when two tests conducted with a 24-hour interval show negative results.
Eun-kyeong says researchers are now conducting a "comprehensive study" into the reactivation of the virus.
The news has sparked concern as some countries were hoping those who had already caught the virus would develop immunity and prevent further waves of the pandemic, Bloomberg says.
There have now been over 1.6 million cases of coronavirus worldwide with 102,000 deaths. 376,000 cases have recovered.
South Korea currently has 10,450 cases, with the daily number of new cases decreasing. The country has 208 deaths and 7117 people have recovered.
