South Korea reported on Monday that at least 116 people initially cleared of the new coronavirus had tested positive again, although officials suggested they would soon look at easing strict recommendations aimed at preventing new outbreaks.
Other experts said faulty tests may be playing a role, or remnants of the virus may still be in patients' systems but not be infectious or of danger to the host or others.
According to WHO's guidelines, a patient is free for discharge after testing negative twice in 24 hours.
But Dr. Jeong Eun-Kyeon, director of South Korea's Centers for Disease Control and Prevention, believes that COVID-19 might reactivate in someone who has been considered cured, Bloomberg News reported Thursday.
"The government needs to come up with responses for each of these variables".
South Korea was hailed as a success story after its swift implementation of a mass testing regime halted the spread of the virus and led to a far lower fatality rate than the global average.
South Korean health officials said on Friday that it remains unclear what is behind the trend, with epidemiological investigations still under way.
The US has more confirmed COVID-19 cases than anywhere else in the world and also has the highest toll, with more than 22,000 deaths.
Because it is a new virus, many questions remain unanswered about the COVID-19 coronavirus - and a lot more research and analysis will need to be conducted.
The city of Daegu, which endured the first large coronavirus outbreak outside of China, reported zero new cases for the first time since late February.
The Trump administration has been accused of being slow to respond to the outbreak, although the president now regularly re-iterates that the United States has tested more individuals than any other country.
"The number will only increase, 91 is just the beginning now", said Kim Woo-joo, professor of infectious diseases at Korea University Guro Hospital. In mild cases of the virus, it takes at least 14 days for patients to show symptoms and then be considered "clinically recovered", according to the outlet.
Mr Kim also said patients had likely "relapsed" rather than been re-infected.