UK researchers say lateral flow tests detect the most infectious Covid cases

Rapid lateral flow tests will likely identify the most infectious Covid-19 cases with higher viral loads despite concerns over the overall sensitivity of the tests, Oxford University researchers said on Thursday, as the British government eyes mass testing to ease the current lockdown.

Along with the roll-out of Covid-19 vaccines, the government has cited widespread testing, including lateral flow tests, as a key part of its plans to re-open the economy. Concerns about the tests' accuracy have led some to question the plan, however.

Some scientists have sounded alarm at Prime Minister Boris Johnson's plans, dubbed "Operation Moonshot", saying mass coronavirus testing is likely to be ineffective and expensive.

Lateral flow tests are less sensitive than PCR tests, which are considered the gold standard, but can return results in just half an hour. Lateral flow tests work best among those with higher viral loads - more virus detected in the nose and throat.

Looking at data from more than a quarter of a million people who have taken part in England's test and trace scheme, scientists found that the greater the viral load, the more infectious the person.

Applying previous estimates of the sensitivity of four lateral flow devices to those findings, the researchers found that the tests would detect between 83.7% and 90.5% of cases leading to onward transmission.

"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, Professor of Medicine at the University of Oxford.

The researchers said they could not ascertain most of those who had been infected asymptomatically, however, "which may contribute substantially to onward transmission."