Tracing app risks sending false alerts from neighbours

THE NHS contact-tracing app could give out false alerts as the Bluetooth connections on which it relies can transmit through walls, experts have warned.

Professors from Cambridge said the app could pick up signals from smartphones in neighbouring flats, leading to people receiving “false positive” notifications, telling them to self-isolate unnecessarily. The warnings come as the Government prepares to deploy the NHS app when the lockdown is eased.

NHSX, the health service’s digital arm, has been developing the app that will use the Bluetooth signals on smartphones to log when people come into close contact with each other.

When a user registers symptoms deemed likely to be Covid-19 in the app, it sends out an alert to all those logged as coming into contact with that user, asking them to self-isolate. Bluetooth, invented in the 1990s by mobile company Ericsson, uses radio waves to link mobile devices, such with wireless headphones.

Ross Anderson, a professor of Security Engineering at the University of Cambridge, said that Bluetooth signals can travel through plasterboard and usually have a range beyond the two metres for social distancing.

This could mean the NHS app wrongly tells individuals who have been in a neighbouring room to someone with Covid-19 symptoms, or walked past at a safe distance, they need to self-isolate, he warned. In turn, that may ultimately lead to some smartphone users ignoring legitimate alerts, Prof Anderson said.

He told The Telegraph: “Bluetooth wasn’t designed for radio ranging – so if you turn up the volume enough to get someone 2m away, you’re probably going to get people four, six or even 10m away as well. You’re going to get a lot of false positives.”

Prof Jon Crowcroft, from Cambridge University’s Computer Laboratory department, said the technology could be a particular issue for users living in flats with thin walls.