The development may alleviate scientific concern that humans are unable to develop long-term immunity against SARS-CoV-2, the pathogen that causes Covid-19.
In the earlier stages of the pandemic, reports from South Korea, China and Japan claimed that a number of discharged patients had tested positive after recovering, sometimes weeks after being allowed to leave hospital.
But now, a South Korean expert panel has concluded that dead virus fragments were the likely cause of more than 290 people in the country testing positive after recovery for coronavirus.
Oh Myoung-don, who leads the central clinical committee for emerging disease control, said there was little reason to believe these cases had resulted from reinfection or reactivation.
“The tests detected the ribonucleic acid of the dead virus,” said Dr Oh, a Seoul National University hospital doctor, at a press conference on Wednesday.
He explained that the technical limits of PCR tests, or polymerase chain reaction tests, mean they cannot distinguish whether the virus detected in people’s bodies is dead or alive, and can therefore lead to false positives.
“PCR testing that amplifies genetics of the virus is used in Korea to test Covid-19, and relapse cases are due to technical limits of the PCR testing,” he said.
Even after a patient has recovered from coronavirus, fragments of the virus can still be detected in the human body for up to two months, Dr Oh added.
“The respiratory epithelial cell has a half-life of up to three months, and RNA virus in the cell can be detected with PCR testing one to two months after the elimination of the cell,” he said.
The committee also said there was little to no possibility that reinfections would occur due to the antibodies that patients develop.
“The process in which Covid-19 produces a new virus takes place only in host cells and does not infiltrate the nucleus,” Dr Oh said. “This means it does not cause chronic infection or recurrence.”
Professor Karol Sikora, the dean of medicine at the University of Buckingham and former director of the World Health Organisation's cancer programme, described the findings as “perhaps the most important news of the week.”
“A huge boost for long term immunity,” he tweeted on Thursday. “Many scientists were very concerned about those reports.”
The committee’s analysis also comes as South Korea reported no new domestic coronavirus cases for the first time since February, as the country continues to make headway in tackling and containing its Covid-19 outbreak.
The Korea Centres for Disease Control and Prevention (KCDC) said on Thursday that it reported four new infections, all imported cases, taking the national tally to 10,765. The death toll rose by one to 247, while 9,059 have been discharged.
The country reached a peak of 909 daily infections on 29 February, before that figure was brought down into the dozens by late March. The number of domestic cases has repeatedly dropped into single digits throughout April, and has now reached zero.
After reporting the first major outbreak outside mainland China, South Korea succeeded in flattening the infection curve within the space of a few weeks – without enforcing an extensive lockdown – due its policy of “test, trace and contain”.
By the time the WHO urged countries in mid-March to “test, test, test”, South Korea had already developed the capacity to test up to 20,000 people a day.
Mobile technology, GPS phone tracking and camera surveillance were also used to help authorities trace contacts and potential transmission between people, many of whom would not have even known they had Covid-19.
Although concerns remain high of a second surge in infections, restrictions on shops, restaurants, bars and religious services are due to be eased if the country’s caseload maintains a decline, while parks and public gardens are also set to gradually reopen.
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