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 with 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. 

On Thursday, officials said they hope the new app, being developed with Apple and Google, will improve accuracy 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 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. 

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 (see video below) when pilots found it could only identify four per cent of contacts on Apple phones.  

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