On this page, you’ll find up-to-date information about the Ketju project.

Ketju warns of the risk of contagion and alerts you about potential exposure.

What is Ketju?

People walking in public space

Ketju is a smartphone application for digitally identifying users who may have been exposed to the coronavirus, developed for a pilot project in Finland in the spring of 2020. The pilot at Vaasa Central Hospital explored how to use Bluetooth technology to identify chains of contagion anonymously and without compromising on data security.

The pilot’s results and the source code for the application have been published under an open-source license.

The technical and functional exploration and development for the application is driven by technology companies Reaktor, Futurice and Columbia Road as well as information security company Fraktal.

How Ketju works

Join Ketju by downloading the app from an app store and by using it whenever you leave home.

When you spend a sufficient length of time in the vicinity of another Ketju user, the app records your encounter.

The app records the encounter completely anonymously: no personal or location data is recorded or stored in the app.

If you start to exhibit symptoms, you'll receive a referral for a corona test.

A doctor calls you to discuss your positive result. The doctor asks you whether you are a Ketju user.

The doctor provides you with a PIN code. By entering the code in your app, you will share your contagion status, anonymously, with other Ketju users.

The apps in the devices of users you have previously encountered will be alerted to the fact that someone they've encountered has been diagnosed with the coronavirus. The app informs the user of potential exposure to the virus and suggests the user contact a healthcare provider.

Only the user is informed of the possible exposure. Officials have no access to the data on recorded encounters.

Frequently asked


An efficient way to control the spread of the epidemic is needed before the extensive restrictions currently in place in many societies can be relaxed. It would consist of identifying chains of contagion, testing those exposed to the virus, and treating as well as quarantining them if they have contracted the virus.

Many countries already have the capacity for large-scale testing, but tracing the chains of contagion remains a challenge. As volumes grow, scaling manual tracing performed by doctors isn’t feasible. It’s also unlikely that people would remember the people they’ve encountered over a period of several weeks.

Smartphone apps developed to trace chains of contagion enable the digital identification of people who may have been exposed to the coronavirus. They help health officials map out who’ve been exposed and contact them. Apps do not replace tracing performed by health officials but support and optimise existing processes.


Apps can increase the speed and accuracy of tracing chains of contagion, which lightens the workload of healthcare professionals. Apps do not replace the tracing and testing performed by health officials, but rather support the process and help direct testing resources more efficiently. The history of encounters recorded by the app is probably more accurate than one based on a person’s memory.


The applications keep a record of people the user encounters who have installed the same application on their phone. They use Bluetooth-technology to infer the distances between users.

In other words, the applications “shake hands” by using Bluetooth to send each other anonymous identifiers. If the users are close enough to each other for a sufficient period of time, the applications exchange identifiers, and the encounter is recorded. If a user is diagnosed with a coronavirus contagion, he or she enters it into the application. All the people whose encounters with the user have been close enough and lasted a sufficient amount of time are sent a notification regarding potential exposure to the coronavirus. Depending on the chosen approach, the notification can come via the app or directly from health officials.


The Bluetooth-based app does not monitor the user’s location. It keeps an anonymous record of other smartphones with the app installed encountered over the course of the day. In the distributed model, data about the encounters is stored only on the user’s phone and no one has access to them without the user’s express consent.

In the Android operating system, using Bluetooth requires giving access to location data, but Ketju does not record or use this data.


The Ketju app used in the Vaasa pilot does not monitor or record the location of the user’s phone. The app only records encounters between users’ devices and it does it anonymously, solely on the user’s phone. The user can not see their own encounter history. In some app versions, only health officials have access to the data and even they need consent from the user.

In the Android operating system, using Bluetooth requires giving access to location data, but Ketju does not record or use this data.


There are roughly three different models or approaches for digital contact tracing: centralised, decentralised, and a hybrid of the two. The key differences involve how much information regarding app user encounters health officials have access to.

In the decentralised model, the user is not asked to provide any personal information whatsoever and health officials have no visibility into the user’s contact history. If the user is diagnosed with the coronavirus, he or she enters this information into the app, which then sends a notification all recorded contacts from the past two weeks.

In the centralised model, the user provides a phone number in conjunction with the download, installation, and registration of the app. The phone numbers and the user’s history of encounters are recorded in an encrypted and central governmental database. If the user is diagnosed with the coronavirus, health officials have immediate access to the user’s contact history and the telephone numbers of those who have been potentially exposed to the virus.

In the hybrid model, the user can choose to remain anonymous but in the case of potential exposure to the coronavirus tell his or her phone number to health officials. Health officials can then contact the exposed party regarding further measures, like testing or quarantines.


No. Ketju was developed for the pilot in Vaasa.


In May 2020, Vaasa Central Hospital piloted technology that helps identify the coronavirus chains of contagion. The goal of the pilot that volunteer members of the hospital staff participated in was to test the use of Bluetooth technology and its background systems in this context.

In the test environment, hospital staff members played the parts of both citizens and health officials. The participants tested only the functioning of the systems and did not use any personal health data. The contagions used in the pilot were simulated.

The pilot results and the app source code have been published under an open-source license.


The solution used in the pilot was based on a distributed model, where the user is not asked to supply personal information. The user’s encounter history is stored anonymously on his or her phone and it is not directly accessible by the user.


Verna Vuoripuro, Reaktor

+358407563088, verna.vuoripuro@reaktor.com

Arttu Tolonen, Futurice

+358414330243, arttu.tolonen@futurice.com

Executive organizations

In cooperation with