The same has been recorded in China, where some coronavirus patients have tested positive after appearing to recover, although there are no official figures.
In South Korea, the proportion of cases that test positive is low – of the 7,829 people who recovered from the coronavirus, 2.1% tested positive, the KCDC said on Friday. It is not known how many people who have recovered have been retested.
But patients who test positive are still a concern around the world, including in countries like South Korea where authorities appear to have brought the epidemic under control.
KCDC deputy director Kwon Joon-wook said that so far there is no evidence that patients who test positive are contagious, although about 44% of them have mild symptoms.
But he warned that there are still many scientists who do not know the virus, including the problem of naturally acquired immunity.
“Covid-19 is the most difficult pathogen we have encountered in recent decades,” said Kwon. “He is a very difficult and difficult enemy.”
Find remnants of the virus?
For now, the most likely explanation for why people retest positive seems to be that the test detects the remnants of the virus.
The KCDC has reviewed three cases from the same family where patients tested positive after recovery, says Kwon.
In each of these cases, the scientists tried to incubate the virus but could not – it told them that there was no live virus present.
Like many countries, South Korea uses a reverse transcription polymerase chain reaction (RT-PCR) to find the virus. The RT-PCR test works by finding evidence of the genetic information of a virus – or RNA – in a sample taken from the patient.
According to Kwon, these tests can still detect parts of the RNA even after the person has recovered, because the tests are very sensitive.
“This is a possible and very strong explanation,” he said.
The same theory was put forward by one of China’s best respiratory experts, Zhong Nanshan. At a press conference earlier this week, he said that a recovered person can test positive because fragments of the disease remained in his body.
“I am not too worried about this,” he added.
What are the other explanations?
There are other theories as to why patients can be retested positive: there could be an error with the test, or the virus could have been reactivated.
If there is an error with the test, patients may have false negatives or false positives. There are a number of reasons why this could happen, including problems with the chemicals used in the test and the possibility that the virus mutates so that it cannot be identified by the test.
In a public briefing, Kwon said that the tests were unlikely to contain errors. However, he said the scientists had re-examined the patients who tested positive to make sure their positive result was not just a problem with the test. “We need further investigation,” he added.
For now, the KCDC is investigating the remaining cases to obtain a more conclusive response.
The evolution of the results can be frustrating for patients. Jin Kim, who is hospitalized in the South Korean city of Daejeon, tested positive for coronavirus on March 25 – this week he tested negative, but a day later he was positive again. The 25-year-old will have to pass at least two more tests, as he needs two consecutive negative tests to be declared recovered.
Once released from the hospital, the government recommends that he isolate for two weeks.
Can a person who retest positive infect others?
Kwon says there is so far no evidence that a person who has tested positive is contagious, adding: “For the moment, we believe there is no danger of secondary or tertiary transmission. “
It is also a concern hanging over the minds of Americans.
Thursday evening, Dr. Deborah Birx, coordinator of the White House coronavirus response, answered a question about patients tested positive at CNN city hall, which the jury had still not determined if someone who had recovered could still clear strands of infectious RNA.
“This is a question that is still pending – it has not been answered in studies to date, although people are really working on it now and are cultivating the virus and seeing if this potential exists,” he said. she adds.
After patients with coronavirus are declared cured, the KCDC recommends an additional two weeks of self-isolation. In an article published in the BMJ medical journal this week, Sung-Il Cho, professor of epidemiology at the Graduate School of Public Health at Seoul National University, also advised discharged patients to remain isolated or quarantined for some time to ensure that there is detection of the virus.
What does this mean for antibodies?
The number of recovered patients who tested positive for the virus has raised concerns about the functioning of the antibodies in response to Covid-19.
When asked Thursday if it was possible for someone to get re-infected, Birx replied: “In biology, you never mean that it is not possible.”
She said she saw patients with coronaviruses seem to recover and develop antibodies, but there was still the possibility of outliers who had not developed antibodies to the virus. “These outliers still exist, but for now we have no evidence that this is a common thing that we see,” she said.
The KCDC plans to test 400 samples of people who have been infected and recovered to see how much – if any – Covid-19 immunity could give people. Kwon says these tests can take several weeks.
In the end, said Kwon, it comes down to this: “We don’t know much about Covid-19.”
CNN’s Shawn Deng and Jake Kwon contributed to this report.