Via Financial Times

South Korea is desperately trying to find out why scores of cured coronavirus patients tested positive for the disease days later, raising questions over scientists’ understanding of the pandemic.

The Korea Centers for Disease Control this week dispatched a team to Daegu, the country’s fourth-biggest city and the worst affected area from the outbreak, to investigate why 51 people in the city and surrounding areas tested positive again.

Interim findings from the probe are expected to be released within the next week but health officials do not believe the patients were reinfected or remained infectious.

“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 surprise positive tests occurred between two days and two weeks after patients had been released from quarantine. Some had shown symptoms such as fevers and respiratory difficulties, while others were asymptomatic, officials said. Further details were not immediately available.

Jerome Kim, an immunology expert and director-general of the International Vaccine Institute, said experts did not yet have a definitive understanding of Covid-19 and that changes might be required in how health officials assess patient recoveries. 

“There are just some things that we don’t know,” Dr Kim said. “Often when virus infections are waning, you will have intermittent positivity and negativity, particularly at the limit of detection . . . Maybe for infected people the government is going to need to look at that data, and say ‘we need two negative tests or three negative tests in a row over a week’.”

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The investigation has also underscored warnings from the KCDC over the need to remain prepared for further outbreaks despite a downward trend in new coronavirus cases in South Korea.

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Over the past two months the combination of mass testing, social distancing and high-tech contact tracing in South Korea brought under control what was one of the worst outbreaks outside China.

On Friday, officials reported 27 new cases, the lowest in more than a month and down from the peak of more than 900 in late February.

Mr Son said the findings would be the latest in a series of instances where health officials and policymakers had to be ready to adapt quickly to new knowledge of Covid-19.

“We learn new things in the process of curing patients,” he said. “At first, we thought that asymptomatic people don’t transmit the virus to others but we found in mid-February that we were wrong. It is a new experience for us so we try to adjust our response as we find new facts.”