The UK government launched its first test of a coronavirus contact tracing app Monday on a small island off the south coast of England.

Using bluetooth technology, the app will alert users if they have been near someone who has tested positive for the virus. The app, which was developed by the digital technology unit of the National Health Service (NHS), will also upload information to a central database to help public health experts study the behavior of the virus.

Unlike Germany, the United Kingdom has chosen not to use technology jointly developed by Google (GOOGL) and Apple (AAPL) that would only store data locally on individual devices. Instead, the anonymous user data will be encrypted and stored by the health authority in keeping with UK privacy rules.

The government hopes more than half of the 80,000 households on the Isle of Wight will download the app after its launch Monday, senior cabinet minister Michael Gove said on Sunday.

If the test is successful, the app will be rolled out across the country later this month, he added.

Such apps can be highly effective in stopping mass infections, according to Christophe Fraser, an infectious diseases expert at Oxford University who is helping to develop the UK app and has extensively investigated outbreaks of SARS, H1N1 and Ebola. If people are alerted that they may have been exposed to the virus, they can take steps to prevent further transmission.

However, to be really be effective at stemming the virus, at least 60% of the population would need to download and use the app, Fraser told reporters last month.

In addition to the app, the UK government says it wants to hire 18,000 contact tracers in the next few weeks as part of its overall efforts to keep track of the virus once lockdown measures have been eased. Contact tracers will track where an infected person has been and who they may have come into contact with.

The UK app is already being criticized by privacy advocates who have argued that a decentralized approach to managing the data provides greater security against bad state actors “spying” on citizens.

Keeping the data centrally located, others experts argue, protects the app systems from being overwhelmed by hacking attacks, and allows governments to better track and study the spread of the infection.

Matthew Gould, who is leading the app development for the National Health Service, told a UK parliamentary committee on Monday that by centralizing the data health authorities can get insight into important information, like which symptoms are more likely to lead to a positive diagnosis and whether there’s a difference in contagion depending on the time it takes for symptoms to develop.

“If privacy was the only thing that we were optimizing for here, then it may well be that a decentralized approach should be the default choice,” Gould said, adding that the NHS is developing the app in consultation with the UK’s Information Commissioner. “It was our view a centralized approach gave us the potential, even when not getting personal data [from app users], to get some very important data that will give serious insight into the virus that will help us.”

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