It is not clear why the NHS app was so much worse as using Bluetooth to detect other phones than the Apple/Google technology is.
And officials have not explained why or how it is better at measuring the distance between two phones.
The main difference between the two apps is the way they store data.
Both keep a log of who someone has come into close contact with – but the NHS’s app would have kept information in a centralised database, while the Google/Apple app is de-centralised.
NHS app: Lists on NHS servers
The NHSX app would create an alert every time two app users came within Bluetooth range of one another and log this in the user’s phone.
Each person would essentially build up a list of everyone they have been in ‘contact’ with. This would be anonymised so the lists were actually just be numbers or codes, not lists of names or addresses.
If someone was diagnosed with the coronavirus or reports that they have symptoms, all the app users they got close to during the time that they were considered infectious – this will vary from person to person – would receive an alert telling them they have been put at risk of COVID-19 – but it wouldn’t name the person who was diagnosed.
NHSX insisted it would have deleted people’s data when they get rid of the app, but not data uploaded to the NHS server if they or a contact tested positive.
Apple/Google: Contained on phones
In Apple and Google’s de-centralised approach, meanwhile, the server and list element of this process is removed and the entire log is contained in someone’s phone.
That app works by exchanging a digital ‘token’ with every phone someone comes within Bluetooth range of over a fixed period.
If one person develops symptoms of the coronavirus or tests positive, they will be able to enter this information into the app.
The phone will then send out a notification to all the devices they have exchanged tokens with during the infection window, to make people aware they may have been exposed to COVID-19.
The server database will not be necessary because each phone will keep an individual log of the bluetooth profiles someone has come close to. These will then be linked anonymously to people’s NHS apps and alerts can be pushed through that even after the person is out of bluetooth range.
It is understood that if someone later deletes the Google/Apple app and closes their account their data would be erased.