BERLIN (Reuters) – Germany on Tuesday becomes the latest European country to launch a smartphone app that seeks to break the chain of coronavirus infection by tracking encounters between people and issuing a warning should one of them test positive.

A growing number of countries in the region have opted to use Bluetooth short-range radio to measure the risk of exposure, after concluding that tracking people’s movements using location data would be intrusive.

European Union member states hope soon to agree a common approach for an international ‘roaming’ feature that could help revive travel and tourism.

WHAT’S THE STORY SO FAR?

Since there is no cure for COVID-19, governments have turned to technology to create a sort of digital ‘herd immunity’ against the flu-like disease.

After initial efforts misfired, Apple (AAPL.O) and Alphabet’s Google (GOOGL.O) – whose iOS and Android operating systems run 99% of the world’s smartphones – developed a standard that logs contacts securely on devices.

Germany joins a growing list of European countries – led by Switzerland, Italy, Poland and Latvia – to create Bluetooth apps based on this decentralized approach.

HOW DOES THE GERMAN APP WORK?

Germany’s Covid-Warn-App, developed by SAP (SAPG.DE) and Deutsche Telekom (DTEGn.DE), will be available for download on Monday night from the Apple App Store and Google Play Store.

Once the app is installed it will typically show a ‘green’, or safe, status.

Should the user spend more than 15 minutes within two meters of another app holder who later tests positive for COVID-19, they would receive a notification advising them to seek medical advice.

WHAT ARE THE SECURITY FEATURES?

The Bluetooth exchanges logged securely on devices are encrypted and pseudonyms are used, so the identity of the other person is not known.

In another security feature, positive test results would be uploaded by the German app using a QR code from the lab.

Other phones scan the system and, if one finds a so-called infected key in its log, the holder receives an exposure notification. No data is stored centrally, making it impossible to reconstruct an individual’s relationships.

SOUNDS COMPLICATED – WILL IT WORK?

The design of Bluetooth-based apps represents a trade-off between usefulness and privacy. It is not possible, for example, to pinpoint the exact time and place of risk events from the app alone.

Protecting privacy, though, is a key selling point for the German app, which is voluntary and will need to be adopted by a large share of the population to be useful.

Norway on Monday halted its COVID-19 app after the country’s data protection watchdog objected to the app’s collection of location data as disproportionate to the task, and called for a Bluetooth-only approach.

Germany’s app is intended to complement, not replace, existing contact tracing efforts that rely on interviewing people who fall ill with COVID-19 and calling people they have met.

Where the app can come in useful is in public settings – such as a train trip or bus ride – where people don’t know each other. Its speed is also a plus as COVID-19 can be spread by people who have yet to develop symptoms.

WHAT ABOUT FOREIGN TRAVEL?

Although the apps are national, the idea is that they should be able to ‘talk’ to each other. Such interoperability would make it possible to monitor infection risks when people travel between countries.

EU member states have already agreed on broad guidelines for interoperability, and are close to backing the creation of a central gateway to handle data traffic between the apps based on the Google-Apple standard, sources say.