One in Three who Isolated Were Never Contagious, Study on PCR Threshold Data Suggests

Up to a third of people who tested positive for Covid by PCR test were not contagious and did not need to self-isolate, a new study led by Oxford scientists suggests. The Telegraph has the story.

Up to a third of people who tested positive for coronavirus by Polymerase Chain Reaction (PCR) tests were not contagious and did not need to self-isolate, a new study suggests.
Research led by academics from the University of Oxford found that many laboratories are setting the positivity bar very low, meaning they are picking up people who are “a danger to no one”.
PCR tests work by cycling swab samples through different temperatures to trigger replication, which releases a chemical showing that the virus is present.
The fewer cycles that are needed to detect the chemical, the greater the viral load and the more likely someone is infected.
There is no definitive cycle threshold level for positivity. However, a review by the University of Oxford found that 30 was a good cut-off, because the virus was unlikely to replicate after that – particularly in asymptomatic people. Other groups have suggested around 32 to 33.
However, Freedom of Information requests made by members of the public and compiled by the University of Oxford show that NHS trusts are using vastly different cut-off thresholds, with little regulation from the Government. Some are as low as 25, while others are as high as 45.
The figures also show that between 23% and 37% of people who were told they were positive had a cycle threshold value above 30. For one in 20, it was higher than 40.
Dr Tom Jefferson, co-author and an epidemiologist at the Centre for Evidence-Based Medicine at the University of Oxford, said: “We found that about one-third of people who isolated probably didn’t need to.
“PCR positivity means that you can tell people to isolate and ruin their lives basically, even though in a large proportion of these cases, they are not infectious.
“It’s absolute chaos. The whole regulation of these tests seems to be shambolic.” …
Prof Carl Heneghan, director of the Centre for Evidence-Based Medicine at the University of Oxford, said: “It’s deeply worrying. When you have more virus circulating in the community, the potential for contamination is greater. Studies have shown that people can be easily contaminated with a viral fragment.
“The problem is the panic of the pandemic meant that we developed policies at a speed we’ve never seen before and we haven’t looked at them to find out which are evidence-based.
“Accurately knowing who is infectious is incredibly important for people going about their daily lives, the economy, our social lives and our wellbeing. The impact on the economy is now coming home, and it is clear that we cannot afford to keep isolating people.
“It’s also clear that members of the public were worried about this and submitting sensible Freedom of Information requests, but were met with contradictory responses attempting to explain a system which is shambolic in terms of governance and regulation.”
The study will be published on the website of Collateral Global, a charity committed to researching the impact of Covid measures.

Worth reading in full, and find the study here.