South Korea's 'reinfected' coronavirus patients are false positives

South Korean health authorities reported last month that dozens of patients who had recovered from COVID-19 later tested positive again.

That revelation threatened to complicate efforts to lift quarantine restrictions and to produce a vaccine.

But after weeks of research, they now say that such test results appear to be "false positives" caused by lingering - but likely not infectious - bits of the virus.

So how do ''false positives'' come about?

South Korea uses reverse transcription polymerase chain reaction (RT-PCR) tests, which detect the genetic material of the coronavirus.

Experts say the tests quickly and accurately detect whether a patient has the virus.

But they also pick up old particles of it, which may no longer pose a significant threat to the patient or others.

Seol Dai-wu, Seoul’s Chung-Ang University:

"The RT-PCR machine itself cannot distinguish an infectious viral particle versus a non-infectious virus particle, as the test simply detects any viral component."

So, it's likely that the 350 people in South Korea with so-called ''false positive'' test results STILL had the virus, rather than a second bout of it.

The World Health Organization has said that there was currently "no evidence" that people who have recovered from COVID-19 and have antibodies are fully protected from a second coronavirus infection.

The United Nations agency has even warned governments against issuing "immunity passports" to people who have been infected as immunity could not be guaranteed.