A study by the Oxford University’s Centre for Evidence-Based Medicine (Oxford, England) suggests that the current COVID-19 diagnostic tests could be too sensitive and may be detecting dead coronavirus cells, leading to overestimated infections.

According to the researchers, the tests used to diagnose COVID-19 are so sensitive that they may be indicating people are infected with SARS-CoV-2 even when they had the virus 70 days ago. Prof Carl Heneghan, director of the Centre for Evidence-Based Medicine, believes that eight days after contracting COVID-19, the chances of an infected person passing on the virus to others become zero if there are no symptoms. However, fragments of the coronavirus can still remain in the body for several weeks afterwards. 

Polymerase chain reaction (PCR) tests currently used to detect SARS-CoV-2 have technical limitations as they cannot distinguish whether the virus in the patient is alive or dead. The researchers analyzed 25 studies on PCR tests and found that such tests can suggest people are infected even after their bodies’ immune system has already fought off the coronavirus because these tests only provide a ‘yes’ or ‘no’ as to whether the virus material is in the body. This means such people would test positive even though the coronavirus in their bodies may be harmless and non-infectious.

"After about day eight, you can still find the RNA fragments," Prof Heneghan told BBC Radio 4's Today program. "Some studies have reported up to 70 days that you intermittently shed. You can understand the importance of this, because what you want to do is find those with active infection and not those with the RNA fragments."“Evidence is mounting that a good proportion of ‘new’ mild cases and people re-testing positive after quarantine or discharge from hospital are not infectious, but are simply clearing harmless virus particles which their immune system has efficiently dealt with,” added Prof Heneghan.

Earlier, a team of South Korean researchers had revealed that reports of recovered coronavirus patients testing positive for SARS-CoV-2 infection a second time round were most likely due to dead-virus fragments. They found that 260 people who had recovered and were declared virus-free tested positive again in South Korea but had little or no contagiousness at all. This meant that they were incapable of transmitting the virus to others, based on virus culture cells that all failed to find live viruses in the recovered patients.

“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. 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,” said Oh Myoung-don, who spearheads the central clinical committee for emerging disease control in South Korea.

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Oxford University’s Centre for Evidence-Based Medicine