New NHS contact tracing app has 'false positive' rate of almost 50 per cent

The NHS contact tracing app being developed is right only half the time so far and has a "false positive" rate of 45 per cent, trial data suggests.

The Government has announced pilots of a new contact tracing app for coronavirus, which will be tested on the Isle of Wight and in the London borough of Newham. It follows months of delays after previous attempts to develop an app were abandoned when it was found to be too inaccurate.

Officials said they hope the new app, being developed with Apple and Google, will improve performance levels. The system, already used in several countries across the world, employs Bluetooth to keep an anonymous log when phones are near each other.

The app will allow users to scan Quick Response codes at pubs and restaurants, meaning patrons can be alerted if other customers test positive. It will also alert users to their general risk levels, depending on the postcode they are in and its level of infection. If a user is told to self-isolate, a timer feature will help count down that period.

The technology is supposed to alert users and tell them to self-isolate for 14 days if they have been within two metres of someone who later tested positive for coronavirus for at least 15 minutes.

Testing so far has found that, for every 10 cases that should be detected, seven are while three are missed. But calculations from 100,000 simulations also found a "false positive" rate of 45 per cent – meaning almost half of all the cases being identified as close contacts do not actually meet the criteria.

Health officials said they hoped to improve on the accuracy on this in the next update to the technology, which happens next month. They added that some of those counted as "false positives" could involve people who been in fairly close contact with Covid cases but further away than two metres.

Professor Christophe Fraser, of Oxford University's Big Data Institute, an adviser to the NHS on the app, said: "At the moment, contact tracing is based on getting people to recall who they have been close to for 15 minutes. This [level of performance] is way better than that."

Originally, ministers promised that a contact tracing app would be available in May. But the NHS version was repeatedly delayed, and finally ditched in June (listen to podcast below) when pilots found it could only identify four per cent of contacts on Apple phones.

Trials of the new system – which can idenitify more than 99 per cent of contacts – will launch first on the Isle of Wight, where tests of the original app took place. They will then be extended to Newham, with no timescale promised for national rollout.

Prof Fraser said improvements in contact tracing would be vital to ensure the safe reopening of schools, adding: "We need to step up our game as schools open."

Matt Hancock, the Health Secretary, said: "It's essential we get the country moving again so people can get back to doing what they love. We've worked with tech companies, international partners, privacy and medical experts to develop an app that is simple to use, secure and will help keep the country safe."

Baroness Dido Harding, who chairs the NHS Test and Trace Programme, said: "There is no silver bullet when it comes to tackling coronavirus.

"The app is a great step forward and will complement all the work we are doing with local areas across the country to reach more people in their communities, and work towards our vision of helping more people get back to the most normal life possible at the lowest risk."

A Department of Health and Social Care spokesman said: "Everyone receiving notifications that they have been close to a positive case is at risk of catching the virus – it would be irresponsible to suggest otherwise – and they should take action, self-isolate and get tested."

Northern Ireland has already launched its own app, while Scotland is expected to have one by autumn.