Coronavirus patients who remain positive weeks after diagnosis may harbour dead virus particles that can’t be distinguished from infectious ones in standard tests, scientists in South Korea found.

The so-called Sars-CoV-2 virus dies one to two weeks after infecting and proliferating inside respiratory cells, doctors at the National Medical Centre in Seoul told reporters on Wednesday (April 29). Pieces of the virus’ genetic material, or RNA, may remain in cells and be detected by a nucleic acid test a month or two after infection, underscoring the limitation of testing, they said.

Some Covid-19 patients have continued to test positive over more than a month, said Prof Peter Collignon, a professor of clinical medicine at the Australian National University Medical School in Canberra, who advises the Australian government on infection control.

It’s possible the virus detected in these patients isn’t viable or capable of causing an infection, “but we need better animal models to see if it’s dead or alive”, he said.

“The finding from South Korea fits in with most of the current thinking, but there are still a lot of unanswered questions about the duration of infectiousness,” he said in an interview.

How long an individual excretes infectious virus in their respiratory secretions and faecal matter is key to determining the optimal time an infected person should self-isolate or be quarantined. Previous studies indicate that very ill patients typically remain infectious longer than people who experience only a mild illness.

The Korean Centres for Disease Control and Prevention said earlier this month that the coronavirus may be reactivating in people who have been cured of the illness. At the time, the agency found that about 51 patients classed as having been cured tested positive again. It concluded that, rather than being infected again, the virus may have been reactivated in these people.
There have been more than 200 reconfirmed cases, and there were discussions on how to explain them,” Prof Oh Myoung-don, a professor of internal medicine at Seoul National University, said during Wednesday’s briefing. “I can tell you the possibility of being reinfected after recovery is low.”

The issue of reactivation and reinfection has been difficult to pin down in part due to the unreliability of testing, with results showing positive one day and negative the next.

Fear of re-infection in recovered patients has also emerged in China, where the virus first appeared last December, after reports that some tested positive again – and even died from the disease – after supposedly recovering and leaving the hospital.

Epidemiologists around the world are in a race to find out more about the virus that causes Covid-19. In addition to research on possible reinfections, health experts are also focusing on patients who contract the virus but display few or atypical symptoms.

South Korea has been at the forefront of tracking these cases, and the issue has raised particular concern in China as the country tries to prevent a second wave of infections.

South Korea was one of the earliest countries to see a large-scale coronavirus outbreak, but it has been one of the most effective in controlling the pathogen. One of the world’s most expansive testing programmes and a tech-driven approach to tracing infections has seen South Korea contain its epidemic without lockdowns or shuttering businesses.