People who have downloaded the UK’s contact-tracing app can expect more warnings to self-isolate in an effort to halt the spread of coronavirus.
Gaby Appleton, who leads work on the NHS Covid-19 app, said an update had cut the “risk threshold” which triggers a notification warning to users who may have come into close contact with an infected person.
Appleton and Randeep Sidhu, the app’s head of product, said the update “is expected to increase the number of people asked to self-isolate by the app, having been in close contact with someone who has tested positive.
“We believe lowering the threshold is necessary to reduce the R rate and break the chain of transmission.”
The UK’s contact-tracing app works using Bluetooth signals. When two people who have downloaded the app come into close contact, their phones perform a “digital handshake”. If one person later tests positive for coronavirus, they alert the app which anonymously warns other people who they have been close to.
The developers said that the technology behind the app had since been tweaked to reflect the severity of the pandemic in the UK.
They said the risk score, which is calculated by an algorithm based on how long people spend together in close proximity, their distance apart, and the estimated date of infection, had been reduced from 900 to 120.
Prof Mark Briers, of the Alan Turing Institute, said the app had also been tuned to make it more accurate at judging distance using Bluetooth.
In the update to the app, NHS Test & Trace added it would also end so-called “ghost notifications” from the app. These warnings include telling people about “possible” exposures. It will now only warn people when they must self-isolate.
Prof Briers said: “Now, if the app notifies you, then it will only be for high-risk encounters - in which case you should follow the instructions provided, and help to keep your family, friends, and community safe.”
New #TuringBlog authored by @markbriers, Programme Director for Defence and Security:
Updates to the algorithm underlying the NHS COVID-19 app: Using #BayesianStatistics to further improve the NHS contact tracing app’s performance https://t.co/GiKxxz6cn3#COVID19 #NHSCOVID19app pic.twitter.com/vx0bGdrAEx
— The Alan Turing Institute (@turinginst) October 29, 2020
The tightening of the app’s risk score comes as deaths from coronavirus in the UK topped 300 for two days running. The seven day average of cases is now more than 22,000 per day, according to the Government’s coronavirus dashboard.
According to NHS Test & Trace, 19 million people have now downloaded the contact-tracing app, around 40pc of people who own an eligible smartphone.
However, millions of people remain unable to access the app. iPhone users need to have iOS 13.7 or later installed and Android users need Marshmallow 6.0 to download the app.
The developers are also still figuring out how to successfully combine the different contact-tracing apps that have been developed by devolved Governments. Currently, the UK’s app cannot warn someone if they have come into close contact with someone using Scotland’s app or Northern Ireland’s app. Apps for Gibraltar and Jersey also work on different systems.
NHS Test & Trace said it expected to fix this problem in an update next month.