Coronavirus: Tests could be detecting dead virus cells, research suggests

Patients may be told they’re infectious when their immune systems have long-since dispatched Covid-19

Coronavirus testing in Staffordshire (Getty)

Coronavirus tests could be detecting dead virus cells, leading to overestimates of infections, a study suggests.

The main test used to diagnose the illness in the UK is so sensitive that it may be suggesting people have the bug when, in fact, their bodies have already fought it off, the research says.

That could mean some Britons are being asked to self-isolate despite no longer being infectious. More pertinently, it could also mean entire areas are being put into lockdown on the back of exaggerated numbers.

If proven correct, the theory would also at least partly explain why cases now appear to be rising in the UK, while hospital admissions are falling.

The researchers – from Oxford University’s Centre for Evidence-Based Medicine and the University of the West of England – gathered their evidence by looking 25 studies on the polymerase chain reaction test, the one most widely used in the UK.

The results showed that because such tests only offer a yes or no as to whether virus material is in the body, they can suggest someone is infected when in fact their body’s immune system has already dispatched the virus.

That means such a person would be told they are positive, despite the bug now being both harmless and non-infectious.

Professor Carl Heneghan, of the CEBM, said: “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.”

Writing in The Spectator, he warned of the “dangers of isolating non-infectious people or whole communities” as a result of such potentially misleading data.

He said that, while it was not logistically possible to check every positive test to see whether the virus was still active, there may be a way of reducing potential false readings.

This is because each test swab goes through a number of cycles in a lab to see if virus material is there. The more cycles it takes to discover the virus, the less of it there is likely to be in the body.

If scientists could agree a reasonable cut off point at which they were confident the detected virus was likely to be inactive, it would mean testers could reduce false positives, said Professor Heneghan.

The possible unintended consequence, he admits, would be a small portion of people being told they do not have the virus when, in fact, they do.

The study itself was published on the Medrxiv online journal and is yet to be peer-reviewed.

Public Health England has said it continues to work on ways to potentially refine testing.