False positives will floor PM’S moonshot testing strategy, warn scientists
BORIS JOHNSON was warned by his own scientists that a Moonshot-style mass testing scheme could needlessly send 28million people into self-isolation in just six months, according to newly published official documents.
A paper published by the Scientific Advisory Group for Emergencies (Sage) calculated that mass testing of the population twice a week – even with 99 per cent accuracy – would throw up so many false positives that over the course of half a year 41 per cent of the population would wrongly be forced to self-isolate.
The Sage “consensus statement”, delivered to ministers at the start of this month, warned schools could be forced to close, and large parts of the workforce lose wages over wrong test results.
Mr Johnson committed on Wednesday to carrying out 10 million daily Covid-19 tests – equivalent to testing every resident once a week – under Operation Moonshot.
The strategy, costing £100 million, was defended yesterday by Matt Hancock, in the face of guffaws in the Commons over his insistence that the programme – promising to produce results in 20 to 90 minutes and requiring untested technology – was feasible.
The Sage document says “careful consideration should be given to ensure that any mass testing programme provides additional benefit” rather than investing resources improving the current test and trace system.
The Government’s credibility on testing has already been questioned over growing problems with the much less ambitious test and trace. Testing has been overwhelmed in the past fortnight with a surge in demand prompting the Health Secretary yesterday to warn that people wanting a Covid-19 test could first face “eligibility checks”.
Mr Hancock, who has blamed holidaymakers and schools for putting testing under strain, said “a strong eligibility check” could be built into the system. He told MPS: “With this very sharp rise we’ve seen in the last couple of weeks of people coming forward for tests when they’re not eligible, that is something we’re having to look at.”
The Department of Health and Social Care later said no specific proposals for the introduction of checks had been drawn up, although eligibility requirements had been made clearer on the forms at testing sites.
Further concern over current testing was raised, with new figures showing the proportion of contacts of positive Covid cases being reached is at a record low. Just 69.2 per cent of close of positive Covid cases were tracked in the week ending Sept 2, down from 91.1 per cent when the programme began. Meanwhile, waiting times for test results are soaring with median times for home test results now more than three days (77 hours), compared to 37 hours at the start of July. In total 1.3 million tests have been processed, despite a capacity for almost double that.
The inability to access tests was blamed by head teachers yesterday for ‘ derailing’ the return to schools, with students forced into isolation because they cannot get tests. While a GP complained yesterday that a colleague was off work for eight days because her daughter’s results had been delayed.
Meanwhile, in Telford, roads into the town were blocked after a booking system error directed people from London, Cornwall and elsewhere to Shropshire.
The Operation Moonshot programme will crank up by Christmas, with three million tests a day promised in December and ten million by spring.
But the Sage paper, dated August 31, raises a series of concerns over the accuracy of mass testing, including problems of “cheaper, faster tests”, which are “likely to have lower ability to identify true positives and true negatives”.
Sage, which is chaired by Sir Patrick Vallance, the Government Chief Scientific Adviser, warns: “In populations with low prevalence of infection, mass testing… would result in many individuals receiving false positive results”.
The Sage document says: “In a population with very low prevalence twice weekly tests… would lead to… 41 per cent of the population receiving a false positive over six months (ie the probability of getting at least one false positive in 52 tests)”. That would be equivalent to about 28 million people. The Sage paper suggests about three per cent of the population – 2.1million – could be forced to isolate on any single day.
The scientists suggest that Covid-19 tests averaged a 99 per cent “specificity”, meaning that one test in 100 will wrongly be returned “positive” when it was actually negative. Jon Deeks, professor of biostatistics at the University of Birmingham, said: “Even if you have a test which is 99 per cent specific, so only 1 per cent of uninfected people get a false positive result, if you then test 60million people, we will be classifying a group the size of the population of Sheffield as wrongly having Covid.” He told the BMJ that, in such a scenario, 600,000 people would have to isolate along with close contacts leading to “substantial economic harm and massive need for further testing”.