Preprint suggests tests detect up to 90% of infections that people would pass onto contacts
A row over the effectiveness of lateral flow tests has heated up among researchers after a preprint suggested the tests are effective in detecting the most infectious cases of coronavirus.
Researchers at the University of Oxford used Test and Trace data to investigate why some individuals pass Covid-19 onto their contacts more easily than others, and whether lateral flow tests could efficiently detect those who are most infectious.
Using information from over 250,000 people who have participated in the programme, the scientists found people with more of the virus detected in their nose and throat were likely to be more infectious.
Modelling also showed that lateral flow tests would be effective at detecting up to 90 per cent of the cases that would lead to onward transmission.
While less sensitive than polymerase chain reaction tests, the scientists concluded that lateral flow testing could be a good solution to ensuring people who are highly infectious receive a positive result and realise they need to isolate more quickly.
Previous community use of the quick tests prompted a debate over their effectiveness, with a recent evaluation of a pilot rollout in Liverpool showing that the devices miss over half of Covid-19 infections in asymptomatic people when compared with polymerase chain reaction tests.
But the government subsequently announced that it was expanding the testing method to all local authorities to test people without symptoms, a move greeted with scepticism by many academics, who claim the tests are not fit for purpose.
The preprint from the University of Oxford has reignited the debate.
“We know that lateral flow tests are not perfect, but that doesn’t stop them being a game changer for helping to detect large numbers of infectious cases sufficiently rapidly to prevent further onward spread,” said Tim Peto, a professor of medicine at the University of Oxford and a senior author on the new study.
He said the tests had been “very popular” with staff at hospitals in Oxford, with over 60,000 tests done since November.
“We’ve been able to detect asymptomatic infected staff who would not have been otherwise diagnosed, protecting patients and staff. The tests can be done at home before coming to work with a result available within 30 minutes.”
David Eyre, associate professor at the University of Oxford’s Big Data Institute, who co-led the study, suggested the tests could be a path out of lockdown.
“When the time comes to relax the current lockdown restrictions, by rapidly identifying the most infectious people using these lateral flow tests, we can potentially relax the lockdown much more safely,” he said.
“This would allow people to get back to work, school and their normal activities and still stay safe.”
But not all scientists are convinced by the latest paper.
Jon Deeks, professor of biostatistics at the University of Birmingham, who has previously criticised community rollout of such tests, said given that contact tracing from Track and Trace data is “severely incomplete” and the individuals will have been symptomatic when tested, “this will affect the generalisability of these findings, particularly to asymptomatic mass testing”.
“Whilst the study provides strong evidence that lateral flow tests can be used as a ‘red light’, as those detected by lateral flow tests are most likely to be infectious, the safety of using these a negative value from these tests to ‘green light’ other activities remains a concern,” he said, calling for evaluation of such tests in “real world settings”.
Alexander Edwards, associate professor in biomedical technology at the University of Reading, described the paper as “incredibly valuable” but said “big policy and public health questions remain”.
“What constitutes a tolerable risk if someone is to be released from self-isolation following a test that is positive but with low levels of virus?” he said. “How accurately can a single swab of one individual be assessed to provide that individual a precise likelihood that they might transmit virus?
“At the moment in the UK the aim is to cut overall infection and death rates by reducing transmission as much as possible. Increased testing using faster and more portable lateral flow tests may be able to achieve this, but we still aren’t sure how well.”
He added that it still wasn’t clear how the findings would translate into asymptomatic screening.
“We already know that test accuracy can vary a great deal in different settings and for different people. This variation in accuracy was only briefly considered in the present preprint.
“It remains vital to keep working hard to make best use of all options including lateral flow tests, but we must continue to evaluate how well tests work in different settings, and to make sure new testing systems are checked to confirm they do reduce transmission.”
A spokesperson for the Department of Health and Social Care said: “Lateral flow devices used by the UK government go through a rigorous evaluation by the country’s leading scientists. They are accurate, reliable and successfully identify those with Covid-19 who could pass on the virus without realising. With up to a third of individuals with Covid-19 not displaying symptoms, broadening testing to identify those showing no symptoms is essential.
“Lateral flow devices detect cases with high levels of virus, making them effective in finding individuals who are the most likely to transmit the disease, including those not showing symptoms. More than 40,000 positive cases have been detected using lateral flow tests, preventing onward transmission through the rollout of community and workplace testing across the country.”