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South Korea reported on Friday that 91 recovered coronavirus patients have tested positive for the disease again, raising questions over health experts' understanding of the pandemic.
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.
The reports have also prompted fears the virus may remain active in patients for much longer than was previously thought.
Korean health officials reported Friday that 91 patients thought to have been cleared of the virus had tested positive again, up from 51 people on Monday.
The Korea Centers for Disease Control and Prevention (KCDC) announced it had sent a team to the city of Daegu, the worst hit area, to investigate why patients there were testing positive again.
Some of the patients testing positive again showed no symptoms, while others were suffering from fevers and respiratory issues, according to the Financial Times.
South Korean health officials said it remains unclear what is behind the trend, with the preliminary findings from the investigation in Daegu not expected to be released until next week.
However the KCDC's director, Jeong Eun-kyeong, raised the possibility that the virus may have been “reactivated” in people, rather than the patients being re-infected.
False test results could also be at fault, other experts said, or remnants of the virus could still be in patients’ systems without being infectious or posing a risk of danger to the host or others.
“There are different interpretations and many variables,” said Jung Ki-suck, professor of pulmonary medicine at Hallym University Sacred Heart Hospital.
“The government needs to come up with responses for each of these variables”.
South Korea had previously been hailed as a success story after its swift implementation of a mass-scale testing regime halted the spread of the virus and led to a far lower fatality rate than the global average.
The country had one of the worst outbreaks outside China in the early stages of the coronavirus spread, but the country has brought the situation under control over the past two months by a combination of measures including transparent reporting, mass-testing, social distancing and extensive contact tracing.
On Friday the country reported 27 new cases, its lowest figure since daily cases peaked at more than 900 in late February, according to the KCDC. The death toll rose by seven to 211, it said.
Nearly 7,000 South Koreans have been reported as recovered from Covid-19, the disease caused by the new coronavirus.
The city of Daegu, which accounts for more than half of all South Korea's total infections, reported zero new cases for the first time since late February.
However the new reports of recovered patients testing positive once more has sparked fears of a fresh outbreak.
“We say that a patient has fully recovered when he or she tests negative twice within 24 hours. But the fact that some of them tested positive again in a short period means that the virus remains longer than we thought,” Son Young-rae, a spokesman for the health and welfare ministry, told the Financial Times.
“The number will only increase, 91 is just the beginning now,” said Kim Woo-joo, professor of infectious diseases at Korea University Guro Hospital.
Mr Kim also said patients had likely “relapsed” rather than been re-infected.