The NHS’s contact-tracing app will fail unless sufficient numbers of Android phone users sign up, experts who have examined its trial use on the Isle of Wight have said. They say NHSX, the digital arm of the health service, is relying on an “Android herd immunity” strategy to overcome shortcomings in the app. A critical mass of Android users will be required to ensure iPhone owners remain covered by the app’s contact-tracing ability. If not enough Android users are in any given community, iPhones will eventually stop broadcasting the signals required for the app to work. “The workarounds NHSX on both iOS and Android are using to create a centralised database seem to be fragile, disruptive to users and risk apps not registering contacts when they should,” said Michael Veale, a privacy expert at UCL. “iOS apps are forbidden from using Bluetooth for long after they are minimised. To keep iPhones registering contact events appears to require either the user to constantly remember to reopen and refresh the app or, stranger still, sufficient Android ‘herd immunity’ among app users, where a nearby user of a non-iPhone, if in range, nudges nearby iPhones to not fall asleep, and to keep listening out.” The workaround is required because the NHS took the controversial decision to build a “centralised” app, which sends data about interactions back to the health service to help with modelling the spread of the disease. As a result, the app cannot use tools built by Apple and Google for the purposes of contact tracing. Those tools, due to be released this month, are restricted to apps that operate in a privacy-first “decentralised” manner. Other countries that have made a similar decision to the UK have faced strict limits to what their contact-tracing apps can do as a result. Singapore, which built the first national contact-tracing app, TraceTogether, had to ask iPhone users to keep the screen turned on and the app open as much as possible in order to ensure their contacts were traceable. In Australia, the government admitted on Wednesday that its tracker app, COVIDSafe, “progressively deteriorates” for iPhone users over time. The UK’s app is being trialled in the Isle of Wight, where it is understood to have been downloaded more than 5,000 times in early release, out of an estimated population of smartphone users of around 100,000 and an overall population of 140,000. Sources say they are confident that the proportion of smartphone owners installing the app will exceed 50% after everybody on the island receives a formal invite by letter in the coming days. Experts from Oxford University have said that the contact-tracing app should be installed by 60% of the population for it to be effective, but those involved believe it will provide useful information about the future spread of the disease with a download rate of 20% – with the help of the centralised database. iPhone’s operating system begins to shut down the UK app’s functions an hour or so after last use, until it ends up in “listen-only” mode. Two phones in that limited mode would be unable to carry out their contact-tracing function. As a result, Android phones, which are less aggressive about shutting down unused apps to preserve battery life, will be key to ensuring the contact tracing works. They will “wake up” nearby iPhones, ensuring that the contact tracing works. If even that does not occur, the app has a second fallback, sending a push notification to users asking them to reopen the application, in effect restarting the clock. Jason Kneen, a freelance app developer, says the workarounds mean the app should have a higher chance of success than those in other nations. Analysing the log files produced while the app is running on an iPhone shows that the software continues to run in the background long after it stops being actively used. “If the argument is that it only works in the foreground, that’s wrong. There is some backgrounding going on, using the normal process.” But Veale said: “Such convoluted workarounds will certainly consume more battery and patience than the Apple-Google approach being adopted in other countries,” including Austria, Germany and Switzerland. “If apps become the international norm, this has significant consequences, including at the Irish border where the Republic has pledged a decentralised approach.” To enable cross-border contact tracing, the NHS would probably have to switch approaches, Veale said. NHSX said its app had been rigorously tested and would run in the background on both iOS and Android phones and still remain effective. Users will not be required to have the app running in the foreground for it to work. NHSX offered up few other specifics except to say that testing of the technology would continue. The UK government is not the only one deciding to plough its own furrow. The French government is waging a much louder campaign against Apple and Google, calling on both companies to change their policies so that it does not need to employ workarounds as Britain has done. “Apple could have helped us make the application work even better on the iPhone,” the French minister for digital technology, Cédric O, told BFM Business TV on Tuesday. “They have not wished to do so.”