Yahoo News 'Vast majority' of positive Covid tests taken in schools likely to be false
The "vast majority" of positive tests taken in schools so far are likely to be false, a leading biostatistician has said as the Government U-turned on its policy to confirm lateral flow results.
Official data shows that the positive rate among secondary pupils is around 0.05 per cent, meaning there is a "high risk" that most rapid antigen tests carried out in secondary schools are false positives, according to Professor Jon Deeks.
Of the 3,867,007 lateral flow tests that were carried out on secondary pupils, just 1,805 were positive, according to the latest figures.
Health officials said that less than one in 1,000 results is a false positive overall. However, when the virus is on the wane, there is a higher likelihood of such results, as the number of true cases falls.
The positive case rate in schools equates to one in 2,142. At a rate of one in 1,000 false positives, from 3,867,007 tests you would expect 3,867 false positives.
It comes as the Department for Education (DFE) updated its guidance to say that all positive lateral flow tests taken in schools should be followed up by a confirmatory PCR test. Previously, this was only the case for tests taken by students at home.
The move came after an announcement from health officials that all Covid cases picked up by lateral flow tests must be confirmed by a second check, amid fears over false positives and missed new variants.
Prof Deeks said the DfE's U-turn meant that pupils will no longer have to "unnecessarily" self-isolate following a false positive. He said he was "bewildered" as to why it took the Government so long to come to this position but said it was "better late than never".
When pupils returned to school on March 8, they were offered three lateral flow tests at school during the first two weeks. For the rest of term they will be asked to take two tests each week at home, but schools will also keep some tests on site for pupils and staff to take as well.
The policy is being changed because the cheap mass testing becomes less reliable as overall cases of Covid falls, and does not allow results to be genetically sequenced.
Millions of people, including schoolchildren, are currently offered regular 20-minute tests to identify cases of coronavirus which might be otherwise missed.
The system has attracted much controversy, largely because of the risks of false positives.
Crucially, the results from lateral flow tests cannot be genetically sequenced – meaning they cannot be used to identify cases of variants being imported from South Africa and Brazil.
On Tuesday, health officials said that from now on, everyone with a positive result from such tests would be asked to confirm the result with a standard PCR test.
And they said the NHS Covid-19 app would soon be updated, so that results from such PCR tests would “overturn” that from a lateral flow test, meaning that any found to be “false positives” could be freed from “self-isolation” policies.
The changes mean that contact tracing will still be triggered by a positive result from a lateral flow test. But in future, this process will be halted after receipt of a negative confirmatory PCR test – if it was taken within two days of the lateral flow test.
Until now, those who undergo lateral flow tests at testing centres, which check for asymptomatic cases, have been expected to isolate if a positive result is found, without further checks.
The change means anyone testing positive under the rapid lateral flow tests – and their households – will still be expected to isolate immediately. However, they could be released from isolation if PCR tests prove negative.
Dr Susan Hopkins, strategic response director for Covid-19 at Public Health England, said the lateral flow tests being used in homes, workplaces and community testing centres have an “extremely low” number of false positives, with less than one in 1,000 such cases found.
She said: “However, as the prevalence of Covid-19 reduces to low levels in the population, the chance of a false positive result increases – though still remains unlikely. During periods of low virus prevalence, we can mitigate against the risk of false positives by confirming a positive lateral flow test with another test – such as a PCR test performed in the laboratory.”
“This makes it highly unlikely that anyone would need to self-isolate unnecessarily, while ensuring that we can retain the benefits of rapid testing to detect true positive cases and for confirming when people are not infected.”
When the tests were first introduced, they were backed up with PCR tests, but this was stopped in January, as prevalence of the virus soared.
Dr Hopkins said the return to the previous policy would allow the system to detect new variants, which cannot be identified through lateral flow tests.
She said: “We are introducing new technology in our labs to identify variants of Covid-19 so we can track the variants that emerge and spread in coming months. We can only do this with the samples taken for PCR.”
In a statement, the Department of Health and Social Care said: “As set out in the Government’s roadmap, our national journey out of lockdown is reliant on us being able to control variants of concern.
“To increase our preparedness and heighten our defences against new variants, we have backed new technology, known ‘genotype assay testing’, which could halve the time it takes to identify if a positive Covid-19 test contains a variant of concern. This will allow positive cases to be traced sooner and stop the spread of variants on UK soil.
“Genotype assay testing is compatible only with PCR tests and in order to detect the maximum number of cases with variants, the Government is reintroducing confirmatory PCR testing for positive LFD test results in England.”
The proportion of pupils attending schools in England last week dropped as more children were forced to self-isolate at home, government data suggests.
Nine in 10 state school pupils were in class on March 25, down from 91 per cent on March 18 when all schools in England had fully returned, the Department for Education (DfE) analysis shows.
The data suggests that 220,000 pupils were out of class and self-isolating on Thursday last week due to potential contact with a case of Covid-19, up from 169,000 pupils in the previous week.
