South Korean patients who test positive for reactivated coronavirus have ‘little or no infectivity’, officials say

South Korean health officials are examining recent research that indicates about half of recovered coronavirus patients retain traces of Covid-19 in their systems after developing immunity, although the risk of transmission to other people remains low.

South Korea on Thursday reported eight new infections, bringing its total number of cases to 10,702. That includes 207 patients who had recovered but again tested positive.

“Patients who have been cured but tested positive again have little or no infectivity,” Korea Centres for Disease Control and Prevention (KCDC) director Jung Eun-kyeong told a briefing.

However, the phenomenon raises a series of questions: whether recovered patients can be reinfected with the virus and whether there is any potential to infect others, which would in turn suggest efforts to develop vaccines based on antibodies’ ability to fight the virus should be redirected.

The KCDC said it was still examining why some patients test positive after recovering. Among the main possibilities are reinfection, a relapse or inconsistent tests, experts said, and Jung has said the virus may have been “reactivated” rather than the patients being reinfected.

South Korea, like many other countries, uses a reverse transcription polymerase chain reaction (RT-PCR) test to find evidence of the virus’ genetic information in a sample taken from the patient.

The KCDC has examined 25 recovered patients to determine whether the virus remains present even after antibodies developed.

“It has turned out all the 25 had neutralising antibodies against the Covid-19 virus,” the KCDC director said on Wednesday, referring to the central part of the adaptive immune system that defends a cell from a pathogen or infectious particle.

“However, 12 people of the 25 people tested positive for the virus’ RNA in [pharyngeal-nasal swab] tests. We assume that it takes different times for the virus to be removed from different patients after antibodies occur.”

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Laboratory cultivation tests using samples from the 12 patients produced no virus, indicating the virus may have already died but the highly sensitive RT-PCR tests picked up its fragments, producing positive results.

Yoo Cheon-kwon at the KCDC said a second cultivation test would verify whether the virus was alive or its genetic remnants were being detected at RT-PCR tests.

“It is highly unlikely that those who tested positive again [in RT-PCR tests] might have been infected again,” he said.

However, health authorities plan to extend self-quarantine for recovered patients beyond the current two weeks because in some extreme cases, cured patients tested positive in RT-PCR tests 35 days after being cured.

Emeritus Professor Lee Hoanjong at the Seoul National University Children’s Hospital said some viruses such as HIV and herpes remain in a patient’s system permanently, although respiratory diseases usually disappear after antibodies form.

“I was somewhat relieved by the fact that the virus were not detected in cultivation tests of the 12 cured patients’ samples,” he said. “Otherwise, this would create much more great problems in combating the disease.”

He said it was unclear whether Covid-19 virus was unique among respiratory disease-causing viruses as there was “simply not enough data”.

Eom Jung-sik at Gacheon Hospital said antibodies failing to eliminate the virus would be cause for concern, undermining efforts to develop an antibody-based vaccine and other treatment.

“It is not the case that all antibodies can control virus and should Covid-19 virus remain alive even after antibodies are formed, it could surge back, causing a relapse in symptoms,” he said. “It would take few years to clarify how efficient neutralising antibodies are in combating the virus.”

Additional reporting by Reuters