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 warned.

They say that 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, then the 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,” says Michael Veale, a privacy expert at UCL.

Contact tracing is one of the most basic planks of public health responses to a pandemic like the coronavirus. It means literally tracking down anyone that somebody with an infection may have had contact with in the days before they became ill. It was – and always will be – central to the fight against Ebola, for instance. In west Africa in 2014/15, there were large teams of people who would trace relatives and knock on the doors of neighbours and friends to find anyone who might have become infected by touching the sick person.

Most people who get Covid-19 will be infected by their friends, neighbours, family or work colleagues, so they will be first on the list. It is not likely anyone will get infected by someone they do not know, passing on the street.

It is still assumed there has to be reasonable exposure – originally experts said people would need to be together for 15 minutes, less than 2 metres apart. So a contact tracer will want to know who the person testing positive met and talked to over the two or three days before they developed symptoms and went into isolation.

South Korea has large teams of contact tracers and notably chased down all the contacts of a religious group, many of whose members fell ill. That outbreak was efficiently stamped out by contact tracing and quarantine.

Singapore and Hong Kong have also espoused testing and contact tracing and so has Germany. All those countries have had relatively low death rates so far. The World Health Organization says it should be the “backbone of the response” in every country.

Sarah Boseley Health editor

“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 specifically 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, which launched its own tracker app, COVIDSafe, the federal government was forced to admit on Wednesday that the app “progressively deteriorates” for iPhone users over time.

Experts from Oxford University have said that the contact tracing app should be installed by 60% of the population for it to be effective in suppressing coronavirus, but those involved believe it will provide useful information about the future spread of the disease around the country – with the help of the centralised database – with a download rate of 20%.

Jason Kneen, a freelance app developer, says a similar effect is visible in the UK app. Analysing the log files produced while the app is running on an iPhone shows that, after an hour or so, the operating system begins to shut down the application’s functions, he told the Guardian, 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 re-open the application, in effect restarting the clock.

Initially only patients in hospital could get tests in the UK. Then testing was expanded to NHS staff and care home staff. Now up to 10 million essential workers and their families who are showing symptoms of coronavirus can apply for a test via a government website

The list of essential workers is the same as the one used to allow the children of key workers to carry on going to school during the lockdown. In addition to health and social care staff, the list includes teachers, judges, some lawyers, religious staff, and journalists providing public service broadcasting.

Also included are local civil servants, police, armed service personnel, fire and rescue service staff, immigration officers and prison and probation staff. Some private-sector staff also qualify including vets, those in food production, essential financial services and information technology, as well as those working in the oil, gas, electricity and water sectors.

Matthew Weaver

The workarounds mean the app should have a higher chance of success than those in other nations, Kneen says. “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 warns: “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 likely be forced to switch approaches, Veale predicts.

NHSX said its app had been rigorously tested and will run in the background on both iOS and Android phones and still remain effective. Nor will users be required to have the app running in the foreground for it to work, although the organisation offered up few other specifics, other than to say that testing of the technology would continue.

The UK government is not the only one deciding to plough its own furrow, though. The French government is waging a much louder campaign against Apple and Google, calling on both companies to actively change their policies so 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.”

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