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South Korea reported Friday that almost 100 patients thought to have recovered from the new coronavirus had tested positive again, sparking fears that populations could become re-infected with COVID-19.

The Korea Centers for Disease Control and Prevention announced that 91 people who had previously been cleared of the virus had tested positive. The KCDC had said the number had risen from 51 cases reported earlier in the week.

KCDC director Jeong Eun-kyeong said Friday that health investigators were still working to determine whether the patients had been “reactivated” rather than being re-infected.

“While we are putting more weight on reactivation as the possible cause, we are conducting a comprehensive study on this,” Jeong said, according to Reuters.

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“There have been many cases when a patient during treatment will test negative one day and positive another.” Tweet This

The question about reinfection also arose last month after health officials in Japan said a woman who had been declared virus-free had tested positive again.

Although uncommon, some viruses stay dormant inside host cells until they’re reactivated. Chickenpox, for example, can occur in children but can later reactivate in adults as shingles.

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While epidemiological investigations are continuing into the issue, health experts in Canada say they are monitoring the situation closely.

David Kelvin, a professor of microbiology and immunology at Dalhousie University, said reinfection is “unlikely” and there could be several possible explanations for the new cases, including that the individual never completely cleared the original infection or the use of faulty test kits.

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“Even with SARS-1, there were patients where the virus lingered on for a very long period of time,” Kelvin said. “If it’s true that people are re-infected, we’re in for a really difficult time. I find it hard to believe, but it’s possible.”

Kelvin said the cases could also be simple false positives or an instance where remnants of the virus could still be in patients’ systems but not be infectious to others.

Whether or not people who have COVID-19 can be re-infected is currently being studied closely by health experts. As there is currently no vaccine, officials are hoping that as populations become infected and recover they will develop sufficient immunity to prevent further pandemics.

“On the good news side, there are a number of companies across Canada that are producing vaccine candidates,” Kelvin said, including at Dalhousie University, the University of Toronto, the University of Saskatchewan, and the private company Medicago in Quebec. “If we could give immunity to our population then we could return to normal.

“In between now and then we have to buy ourselves time with rolling lockdowns.” Tweet This

Anthony Fauci, the director of the U.S. National Institute of Allergy and Infectious Diseases, told the Journal of the American Medical Association this week that reinfection was unlikely.

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“If a person gets infected with coronavirus A, and then gets reinfected with a coronavirus, it may be coronavirus B,” Fauci said. “But right now, we don’t think that this is mutating to the point of being very different.”

South Korea was one of the earliest countries to see a large-scale coronavirus outbreak, but the country has seen just 200 deaths, due largely to widespread testing and advanced contact tracing.

The country has had over 10,000 virus cases, with more than 6,700 released from hospital, according to data compiled by Johns Hopkins University.

Canada, meanwhile, has seen more than 21,000 infections and over 500 deaths.

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Kelvin said there have been few studies done on reinfection, but they have so far found it’s unlikely that people can get the novel coronavirus more than once.

He pointed to one small study from China that found antibodies in rhesus monkeys kept primates that had recovered from COVID-19 from becoming infected again upon exposure to the virus.

“They found there was no reinfection,” Kelvin said. “Whatever the measurement countries are using for reinfection, we have to make sure it’s a bona fide test kit.”

—With files from Reuters