An increasing number of countries are partially lifting the acute social isolation measures taken at the beginning of the COVID-19 pandemic. However, the global infection rate is still constant at almost 100,000 new cases per day [1]. With the need to return to some sort of normality [2] but with whole-population-testing being impractical, politicians and experts are looking at smartphone apps as tools to make virus countermeasures more targeted and effective.

In general, the usage of such apps is voluntary, and the proposed apps can be divided into survey apps and contact tracer apps.

The aim of survey apps is to estimate whether an individual is infected with SARS-CoV-2 via a series of questions regarding the user and their symptoms. Together with location data, this data is pseudonymised* and shared with a central database. Scientists can then use this data, for instance to model where the epicentres of the spread are and how they develop [3]. This information is crucial to guide effective countermeasures and, yet, difficult to obtain on the same scale with classical methods [4]. However, since the diagnosis of COVID-19 is only based on user inquiry and not on medical tests, it is only effective on a population level and not as a test for individuals [5]. Moreover, scientists face several statistical challenges when working with app-based data, such as not having control over the social groups that are contributing to the data – e.g. infants and elderly are most likely underrepresented [5].

Contact tracer apps are intended to guide self-isolation for individuals by informing them if they have been in contact with a COVID-19 positive person. This is achieved in the following way: The app-user has the app installed on their smartphone. When they come into close contact with another app-user, their phones log this by exchanging user IDs via Bluetooth. If one of the users later reports a positive COVID-19 test, the other user is notified and asked to self-isolate. It is important to note that no information about the identity of the positive user is shared in this notification. In addition to allowing for ‘informed isolation’, this system also allows researchers to track the spread of the virus in detail and it has been shown that contact tracing can be an effective tool for epidemiologists [6]. However, even though theoretical studies show that contact tracer apps can be an efficient tool against the spread of COVID-19 [7], user participation and tracing proximity precision are major challenges that need to be overcome for such apps to be effective [8].
Moreover, due to the personal and sensitive nature of the collected data, contact tracer apps need to adhere to the highest data protection standards [9]. In this context, these apps can be categorised in ‘centralised’ and ‘decentralised’ architectures. Centralised contact tracer apps share the user’s health status and all contacts with other users with a central database, which computes whether a user was in contact with a positive user and reports only this information. Decentralised apps store user contacts locally (on the smartphone) and only share the health status with the database, which in turn gives access to a complete list of all users’ (pseudonymised) health status. The app then computes potential contacts with positive users locally. Data safety experts strongly recommend decentralised contact tracer apps [10, 11].