Coronavirus: NHS contact tracing app to target 80% of smartphone users

A contact-tracing app could help stop the coronavirus pandemic, but 80% of current smartphone owners would need to use it, say experts advising the NHS.

The University of Oxford's Big Data Institute has modelled a city of one million people to simulate the software's impact.

If there is lower uptake, academics say the app would still help slow the spread of Covid-19.

They add that letting people self-diagnose the illness could be critical.

That means users would only have to answer an on-screen questionnaire before being judged to be at significant risk of infection. They would not have to speak to a health advisor or wait for a medical test result.

This would send a cascade of alerts to people they had recently been in proximity to, advising them to go back into self-isolation.

The experts say "speed is of the essence", and that delaying contact tracing by even a day from the onset of symptoms could make the difference between epidemic control and resurgence.

"There would be more people receiving notifications as a result of false warnings," explained Prof Christophe Fraser.

"But actually, it results in fewer days of people in self-isolation and quarantine, because the effect of suppressing the epidemic more quickly outweighs the risks in waiting for a test before the notification."

The over-70s have not been factored in, on the basis they would remain "shielded" by staying at home, he added.

The Oxford University academics are a mix of epidemiologists and ethicists advising NHSX - the health service's digital innovation unit - on what basis the contact-tracing app should be created. They are not involved in coding or designing the software itself.

Their model takes into account different age groups, household structures and movement patterns in an effort to try to maximise the number of people who could be allowed to freely move about once a contact-tracing app has been launched.

"We're looking at introducing the app towards the end of lockdown," Prof Fraser added.

"When you install it, it needs a few days to start recording data before it can be fully functional."

The group first published a paper about its work at the end of March.

Since then, they have adjusted their model to take account of changing factors, including the fact that Covid-19's infection rate has been faster than they had anticipated.

They have also changed plans from using a system that relied on GPS location readings and scanning QR codes to one that exclusively depends on Bluetooth signals. This has been done to provide users more privacy, which in turn could encourage take-up.

The hope is that using the app, as well as other measures such as hand-washing and social distancing from vulnerable members of the population, will prevent a second peak in infections or the need for repeated national lockdowns