If the UK doesn’t change its disastrous contact tracing app strategy, Northern Ireland should align with the Irish government approach

As trials of the British government’s coronavirus contact tracing app begin this week on the Isle of Wight, serious questions have been raised about the design of the application and its potential effectiveness.

There are two broad categories of contact tracing app: the centralised approach, where data from phones is sent to a central database where contact data can be analysed, or the decentralised approach, where in order to protect user privacy contact data is analysed on phones without the need for a central database.

Unlike most countries, the UK is going down the path of having a centralised system, whilst the Irish government is advocating the decentralised approach.

Theoretically, there are arguments for either approach, from the perspective of the trade-off between privacy and the government having richer data through which to analyse the spread of the virus. However, in practice the centralised approach advocated by the British government is likely to be vastly less effective than the decentralised approach.

Apple and Google, who release the iOS and Android operating systems used by virtually all modern smartphones, are only allowing decentralised contact tracing apps to use the Exposure Notification API. Contact tracing apps that use this API are allowed to transmit to other nearby phones using Bluetooth even whilst users don’t have the contact tracing app in the foreground, or whilst a phone is locked.

Therefore, phones using the UK government app will generally only be able to transmit to other phones if they are unlocked and the app is being used, whilst apps using the decentralised approach such as the Irish government app will be able to register contacts with other users even if both phones are locked or the app is in the background.

There are a couple of exceptions to this rule for centralised contact tracing apps. Phones running older versions of Android (version 7 or earlier) will be able to transmit whilst the phone is locked, and newer Android phones will be able to transmit for “several minutes” after the app is put into background mode. However, phones running iOS will not be able to transmit to other nearby phones at all unless the app is in the foreground.

Whilst phones running the UK government app will be able to “receive” messages at any time, a contact will only be made if there is another phone running the app in the foreground, or an older Android phone, in the immediate vicinity to transmit a message to tell the app to wake up. For example, if three people carrying an iPhone are standing close to each other, the UK app will not register a contact. However, if they were joined by a person carrying an Android 7 phone, the older Android phone would wake the others up and they would all register contacts with each other.

Roughly 77% of UK adults use a smartphone, of which 52.3% run iOS and 47.5% run Android. Amongst Android phones, only 24% run Android 7 or earlier and will therefore be able to run in “transmit” mode even when the phone is locked. If we allow that iPhones and newer Android phones will be in transmit mode 1% and 2% of the time respectively, if 100% of smartphones had a contact tracing app installed, then the probability of a random adult having a smartphone operating on “transmit” mode at any given time is 7.7% if they are running the UK centralised approach app, and 77% if they are running the Irish government’s decentralised app.

Of course, a 100% uptake of the software amongst smartphone users is unrealistic. The country with the highest incidence of contact tracing app installs, Singapore, currently has a 20% uptake rate. The Australian government is aiming for 40%.

The chart below shows the probability of at least one adult carrying a smartphone on “transmit” mode, if there was a group of x people (including you), with 40% uptake amongst all adults of the contact tracing app.

Therefore the probability of you receiving a contact notification if you are in a group of x adults, one of whom subsequently tests positive for Covid-19, with centralised and decentralised apps is shown in the chart below.

The infographic at the top of the page illustrates the probability of receiving a contact notification for groups of size 2, 4 and 10 if one person subsequently receives a Covid-19 diagnosis.

It is worth noting that this analysis assumes that all people have an equal probability of installing the contact tracing app. In reality, it could well be the case that less tech-savvy people are more likely to have the vital older versions of Android (Nougat, Marshmallow, Oreo, Lollipop, and older) needed for the system to work, but also less likely to install the app in the first place.

The proposed UK system will rely heavily on many people installing the app on Android phones that are more than three years old. If the UK decided to adopt the decentralised approach, it would be able to avail of the Exposure Notification API, and these issues would not exist. The percentage of people running older versions of Android will naturally fall over time as people upgrade their phones.

Whilst Google and Apple generally only allow one contact tracing app per country, flexibility is being shown where one country can have different systems, for example different states in the United States. If the UK government cannot be dissuaded from pursuing their current approach, then Northern Ireland should follow the much more effective strategy of using a decentralised approach and align with the Republic.

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