While accounting for only a small fraction of the more than 7,500 discharged patients in the country, the phenomenon has raised important questions for public health officials around the world who are struggling to understand the virus behind the respiratory disease Covid-19, which has claimed over 130,000 lives and halted commerce and travel worldwide.

Scientists say that likely explanations – ranging from the relatively benign to those which raise major public health concerns – include the fallibility of testing, the effect of “intermittent shedding” of the virus by patients, and, more seriously, the virus being “reactivated” after a period of dormancy.

Although it is possible, many experts are sceptical about whether the trend represents cases of actual reinfection – which would raise serious doubts about the ability to develop a vaccine – based on existing knowledge of other coronaviruses, which people are generally immune to for at least a year after infection.

KCDC director Jeong Eun-kyeong is among those to have suggested the virus “being reactivated” as a possible explanation for the apparent relapses, although the agency is yet to reach any definitive conclusions.

It is not unheard of for a virus to “hibernate” in a patient for a prolonged period of time or remain active in certain parts of the body and not in others. Ebola can remain sexually transmittable in a patient after recovery and chickenpox contracted as a child can re-emerge years later during adulthood as shingles.

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Kim Woo-joo, an infectious diseases expert at Korea University Hospital, said the possibility of mutation was a major concern raised by the apparent relapses.

“About one fifth of them are healthy people with a good immune system but they still tested positive for the virus after they were initially diagnosed as cured,” Kim said. “Researchers are testing blood samples to determine whether patients are reinfected because they have problems with their own immunity system or the virus has mutated somehow to circumvent the bodily defence system.”

Although the KCDC has yet to release detailed information on how many of those who tested positive a second time were symptomatic, which is a key concern for public health experts, at least some of them are reported to have had mild symptoms such as fever. Authorities have yet to report any cases of patients testing positive twice infecting others.

US-based infectious diseases expert William Schaffner said it would be of particular concern if the coronavirus was reactivating in patients while accompanied by symptoms.

“It raises the question … whether the so-called treatments can provide assuagement of an illness – make you feel better – but are not capable of eliminating the virus completely from the body,” said Schaffner, who is a professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine in Nashville, Tennessee.

Schaffner said recurring illness would place additional strain on health systems and also require follow-up with patients after their discharge “to determine as early as possible if a recurrence would occur so we can take care of them properly”.

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“We’ve been coping more or less with the acute phase and we haven’t focused on follow-up of patients at all,” he said. “This could radically change what it is that we are doing.”

Cho Sung-il, a professor of epidemiology at Seoul National University, described the possibility of recurring illness as “unlikely” but “very disturbing” as it would suggest patients could become infectious again after being discharged.

Cho said testing by the KCDC could provide further clarification within days.

Ian Frazer, an immunologist at the University of Queensland in Australia, said an important variable that remained unknown was the length of time between recovery and retesting in each case, with shorter periods counting against the possibility of reinfection. South Korean authorities have said that patients who tested positive twice did so “shortly” after their release from quarantine.

“Given the data from animal studies, more likely there was residual virus rather than reinfection,” Frazer said.

Michael Kinch, director of the Centre for Research Innovation in Biotechnology and Drug Discovery at Washington University in St Louis, US, said it would be “unusual” to see relapse in the case of a virus like SARS-CoV-2.

“You do see such a thing with HIV, but that is [because] that particular virus integrates into the host’s genome,” said Kinch. “I would anticipate that the ‘reactivation’ is simply reinfection. Contrary to the presumption that infection will automatically lead to immunity, this is often not the case.

“If SARS COV-2 behaves like its less virulent predecessors, then infection may not presume protection,” Kinch added. “This could have dire implications for a vaccine.”

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Kim said that although less likely, reinfection was a possibility.

“In two to three weeks after infection, the body produces antibodies. But this does not always occur,” he said. “For example, about five per cent of people who are vaccinated against hepatitis B do not produce antibodies in their blood. And some people infected with the SARS-CoV-2 virus produce no antibodies against the agents and they can be reinfected by the virus.”

“If I would hazard a guess, I think that these cases are not real relapses or reinfections, but represent prolonged intermittent shedding of SARS-CoV-2, which is seen in other viruses such as influenza as well,” said Hsu Li Yang, an associate professor at Saw Swee Hock School of Public Health at the National University of Singapore. “If so, there is no impact on treatment or the control of the outbreak. We will need more clear-cut evidence and studies to be sure, however.”

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Experts widely agree it is too early to draw firm conclusions about the situation in South Korea. At the same time, many anticipate that the wealth of data emerging from the country’s mass testing will include answers to this and other questions about the virus in time.

“South Korea has done an excellent job with epidemiologic surveillance,” said Howard Forman, a professor of public health policy at Yale School of Management in the US. “I am counting on them to clarify this soon.”

Schaffner said this emerging trend was more likely to be picked up because “particularly in Korea, they are testing so assiduously, so comprehensively”.

“With this whole epidemic … Asia has been ahead of the Western countries,” he said.

Additional reporting by Park Chan-kyong