By Leo Kelion & Rory Cellan-Jones
People living in England and Wales are being urged to download the government's official contact-tracing app following its official release.
NHS Covid-19 instructs users to self-isolate for 14 days if it detects they were nearby someone who has the virus.
It also has a check-in scanner to alert owners if a venue they have visited is found to be an outbreak hotspot.
Anyone over the age of 16 is being asked to install the app onto their smartphone.
That is a change from trials, which were limited to the over-18s.
The move reflects a desire by health chiefs for the software to be used by as many students in further education colleges and universities as possible.
The age limit is in line with the Protect Scotland contact-tracing app. And health chiefs behind Northern Ireland's StopCOVID NI have said they intend to launch a new version that accepts under-18s later this month.
The launch comes as the UK reported 6,178 coronavirus cases on Wednesday, up 1,252 since Tuesday, and 37 deaths.
Health Secretary Matt Hancock said that with coronavirus infection rates rising, the app could help keep people safe.
"I urge everyone who can to download and use the app to protect themselves and their loved ones," he said.
The government had originally intended to release the app months ago. But problems with the initial design and the addition of extra features meant it was only ready for its final public test in August.
One tech expert who has tracked the initiative acknowledged the team involved had worked hard to address concerns about privacy and transparency, but said wider problems could still limit its impact.
"Not only is the app late to launch, but it will be hindered by the delays in the testing system," Rachel Coldicutt told the BBC.
"If you don't have symptoms, will a push notification saying you were near someone a week ago make you and your family self-isolate and spend days hitting refresh on the testing website, trying to find a test?"
Although the app allows users to order a coronavirus test and automatically get the results, the government does not believe it will complicate efforts to meet demand.
How can people access the app?
The app is available for smartphones only – not tablets, smartwatches or other devices.
The handsets must have Android 6.0 (released in 2015) or iOS 13.5 (released in May 2020) and Bluetooth 4.0 or higher.
And some of the latest Huawei handsets are excluded.
What will contact-tracing alerts say?
The notification will tell the recipient to go into self-isolation for a fortnight – and trigger the start of the app's countdown clock.
Even if the recipient has no symptoms or a subsequent negative test result, they must stay at home for the duration.
Unlike when a human contact tracer orders someone to self-isolate, the app keeps the subject's identity a secret.
Those that have not received such an alert, but fill in the app's symptoms checker and meet the criteria, will be directed to self-isolate for eight days and be directed to an external website to book a test.
Automated contact tracing is designed to complement the work done by humans by helping to identify encounters with strangers – for example, someone standing close to a user while waiting in a queue outside a shop.
When someone anonymously shares a positive Covid-19 test result via the app, a process developed by Apple and Google makes other users' smartphones check if they had recently detected the infected person's handset.
But smartphones were not designed for this purpose and the readings involved are not always accurate. This has led to fears of "false positives" – people being instructed to self-isolate who were never at risk.
However, health chiefs believe a recent change to the calculations involved should help minimise the problem: they now take into account when the virus-carrier was at their most contagious.
The Department of Health has confirmed that users who ignore the app's self-isolation warning are in theory liable for fines of £1,000 or more.
However, because they do not have to identify themselves, officials acknowledge this will normally be impossible to enforce.
One of the app's other major features is a scanner to let it keep track of pubs, restaurants, hairdressers and other places the user has visited.
Hospitality venues in England now face £1,000 fines if they do not display an official Test and Trace poster featuring a QR barcode assigned to them.
So far, the government says more than 160,000 businesses have downloaded a unique code for their property.
The benefit is that if officials latRead More – Source