Ninety-one recovered coronavirus patients have tested positive for the disease again in South Korea, raising questions over health experts' understanding of the pandemic.
The prospect of people becoming infected for a second time 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 also suggest the virus may remain active in patients for much longer than was previously thought.
The Korea Centres for Disease Control and Prevention (KCDC) has sent a team to the city of Daegu, the worst-hit area, to investigate why patients there are testing positive again.
Preliminary findings are not expected until next week, but Jeong Eun-kyeong, the KCDC director, raised the possibility the virus may have been "reactivated" in people, rather than the patients being reinfected.
False test results could also be at fault, other experts said, or remnants of the virus could be in patients' systems without posing a risk to them 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."
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.
The country had one of the worst outbreaks outside China in the early stages but has brought the situation under control in the past two months.
Yesterday, it reported 27 new cases, the lowest 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.
Daegu, which accounts for more than half of all South Korea's infections, reported zero new cases for the first time since late February.
Reports of recovered patients testing positive once more for the virus 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 some of them tested positive again in a short period means the virus remains longer than we thought," a spokesman told the 'Financial Times'.
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