NEWS -- South Korean health officials reported on Friday that they were investigating 91 cases of COVID-19 where patients who were thought to have recovered retested positive.

Korea Centers for Disease Control and Prevention said in a press release that a cluster of infections at a nursing home found that 18 out of 36 confirmed cases that had been released from quarantine had re-tested positive just days after testing negative.

Generally, an infected patient is considered recovered when they have tested negative twice in the span of 24 hours.

Scientists say there are several potential scenarios for these test results. Possibilities include: the patients were actually infected with a different variation of the virus from the original infection, there were errors in the testing, or the patients never fully recovered. Another possibility is when a virus from the original infection mutates in the body and causes a re-infection, but this is considered rare.

“Dogma suggests if we are infected with a virus the first time and we recover, we have an antibody response to that virus, so we produce neutralizing antibodies that help us eliminate that virus from our system,” Dr. Marc-André Langlois, a molecular virologist and professor with the Faculty of Medicine at the University of Ottawa, told CTVNews.ca in an interview.

“If you have a primary infection and you mount an antibody response, the chances to be re-infected with that same virus is almost nil. This is what happens in a normal response.”

Scientists around the world are currently doing research into antibody testing and how immunity to SARS-CoV-2, the virus that causes COVID-19, could be one path to fighting the pandemic.

If a patient is infected with a variation of a particular virus, the antibodies might not necessarily recognize and neutralize it accordingly.

“It’s a virus that does mutate, but it does not mutate at the same rate, at the same speed as other RNA viruses like influenza or HIV,” Langlois said.

Nextstrain, an open-source project, looks at the genetic data of SARS-CoV-2 to track the virus’ evolution as it moves around the world and how it spreads locally. Genetic sequencing of the virus isolated in different parts of the world - Asia, Europe, and North America - have found variations that can be clustered and traced. The genetic footprint of the virus circulating in New York, for example, can be traced to Europe, according to separate studies conducted by scientists at New York’s Mount Sinai and NYU’s Grossman School of Medicine.

Scientists have said that the genetic variations so far are quite minor, with the different strains being fairly similar to one another.

'REACTIVATED' VIRUS?

Health officials in Korea appeared unconvinced the 91 patients had been somehow re-infected, according to media reports of press briefings held this week. KCDC Director-General Jeong Eun-kyeong told reporters the virus may have been “reactivated” instead, due to the short time frame between when the patients were discharged and when they tested positive again.

THE KCDC said response teams at both the local and national level were investigating the epidemiological and clinical characteristics of these cases, which include examining virus isolation cultures and blood antibody tests.

There have been reports from China as well of patients retesting positive after having recovered. An NPR report last month looked at four such cases involving residents in Wuhan, where the outbreak first began. The patients tested positive days or weeks after they were deemed recovered.

There are still a lot of uncertainties and unknowns around SARS-CoV-2 - in this case, whether the virus detected in the retests are active or dead. This would determine whether the patient can infect others or not. Research indicates that RNA can still be detected in tests even after a patient is no longer infectious.

But in order to see how these patients are testing positive again, Langlois said researchers will need to look at the genetic sequence of the virus in the initial positive sample prior to when they were considered recovered, then look at the genetic sequence of the second test and compare. Scientists would also need to monitor whether the individuals produced antibodies.

“That is the only way you can absolutely ascertain that re-infection occurred,” Langlois said. But sequencing is not consistently done due to the volume of testing being conducted.

“All they want to do is make sure they’re positive or negative so they can triage them properly ... the sequencing is not routine for every single diagnostic case and that’s part of the issue. There’s no capacity to sequence everything. It’s costly and time intensive.”

ROOM FOR ERROR

Errors at the testing level and faulty results could also be the source of the unusual results, scientists say.

A test for COVID-19 is typically done through swabbing the nasal airways - a nasopharyngeal swab - with the sample then placed in a liquid that is used to detect the genome of the virus. The swabbing needs to be deep enough to collect from the areas where the virus is most prevalent, and errors can occur when the swab is not done correctly.

The test uses a polymerase chain reaction (PCR), a process in molecular biology that allows scientists to make millions of genetic copies from a very small sample, amplifying it in enough quantities to study. Different countries and health organizations designed different versions of the test and early in the outbreak, some were less sensitive than others, resulting in faulty results. This is one theory behind the positive retests in China and a possibility for the cases in South Korea.

“These are all areas of questioning - as to are these really re-infections or was something missed the first time?” Langlois said. “My hunch is that it is not a re-infection by the same virus.”