Covid rapid tests are more widely used: can they be trusted?

Britain and France agreed this week to use rapid on-site Covid-19 tests to end a 48-hour blockage on travel between the two countries, the latest adoption of technology that could become a crucial tool to control the spread of the coronavirus in 2021.

The UK government has spent at least £ 1.5 billion on so-called lateral flow tests and has already supplied the simple devices to local authorities, nursing homes, hospitals and universities. The UK regulator went further on Tuesday, granting the tests emergency clearance for use by members of the public without medical training.

Supporters of lateral flow testing say the devices, which can provide a result in 20 minutes, will significantly increase the UK’s testing capacity and are a useful tool in finding people who are currently infectious. From next month, tests will be rolled out to UK schools to identify cases of the virus before the start of the new term.

“Testing asymptomatic people is a huge priority,” said John Bell, Regius professor of medicine at the University of Oxford, who has overseen studies on the accuracy of these tests for the government. “If there isn’t a test, they don’t get caught. Every positive result you get is a victory. “

But several scientists remained skeptical despite the regulator’s approval and increasing adoption, arguing that the public has been misled about the accuracy of the tests.

They’re not perfect, but that doesn’t stop them from being a game-changer to help spot cases

Unlike RT-PCR tests, which look for the genetic material of Covid-19, lateral flow tests identify a protein antigen on the surface of the virus. Lateral flow devices do not amplify the virus (unlike RT-PCR tests) and therefore struggle to detect lower levels of infection.

Several analyzes have shown that the lateral flow test developed by Innova – the UK’s largest supplier and one of the few tests meeting the government’s criteria for precision – detected less than half of active infections.

“Schools are going to be a nightmare,” said Tom Lewis, clinical manager of pathology at North Devon District Hospital. “If you rely on this to collect the stuff, you’re going to miss more than half of it.”

Oxford University professor Tim Peto disagreed. “They’re not perfect, but that doesn’t stop them from being a game-changer to help detect cases of infectious diseases,” said Professor Peto, who worked with Professor Bell to validate the accuracy of the tests.

Their analysis found that Innova tests detect between 84 and 96 percent of infections in people with high viral loads, making them an ideal tool for identifying people who are currently infectious. The study also concluded that using the devices to identify and then quarantine cases of Covid-19 can reduce transmission by 67-90%.

This means that the test is the most effective at catching infectious people and less effective at identifying people who have a low viral load, who are normally at the start or end of their Covid-19 infection. In contrast, “benchmark” PCR tests detect all forms of viruses.

The best source of data for the widespread use of rapid tests to trace infection and reduce transmission comes from Slovakia, where two-thirds of the population – 3.6 million people – were tested between October 31 and on November 1 and over 57,000 cases have been identified.

An analysis of the campaign by researchers at the London School of Hygiene and Tropical Medicine estimated that the regimen of mass testing, coupled with quarantine measures, led to a 60-70% reduction in the rate of infection. in Slovakia.

“We are seeing strong evidence that the combination of short lockout and mass testing led to a very rapid reduction in the burden of Covid-19,” said Stefan Flasche, professor of vaccine epidemiology at LSHTM and co- author of the article.

The largest pilot program in the UK was carried out in Liverpool, where 25% of the city’s 497,000 residents took a rapid test in November, leading to the identification of 897 positive cases, according to full data released Wednesday. The Innova tests used detected 40% of PCR positive cases, two thirds of those detected having a high viral load.

The academics who led the study said the targeted rapid tests were both popular and valuable. Although they could not prove that the tests led to a drop in the infection rate, they noted that cases in Liverpool had declined more rapidly than in neighboring Manchester.

Evidence suggesting that lateral flow tests are able to detect the virus when carriers are infectious suggests that they may be particularly useful for making short-term behavioral decisions, such as entering a nursing home to visit a relative. . However, scientists widely agree that some pre-symptomatic cases will be missed, making it an imperfect tool.

“There is no perfect test, that’s the problem, everything is a compromise,” said Jo Martin, professor of pathology at Queen Mary University. “You have to know its limits when you take it.”

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