The use of apps as a tool for containing the COVID pandemic is currently being intensively discussed. So-called tracing apps are intended to inform contact persons of positively tested persons as quickly as possible so that they can immediately go into quarantine. This is particularly important because in this so-called pre-symptomatic phase, infected persons without symptoms can continue to infect other people. The aim is to significantly reduce the number of contacts that lead to a possible passing on of the infection by informing those affected as early as possible, thereby reducing the dynamics of the spread of the virus. Due to the still low frequency of SARS-CoV-2 infections in the population, the rapid contacting and isolation of infected persons and their contact persons is of great importance, and public health authorities need the most efficient support possible in this respect. Current plans for a tracing app in Germany call for contact persons of infected persons to be identified anonymously via Bluetooth and the identification (anonymous ID) of their app instance to be stored. According to the latest information, a decentralized solution without storing the ID on a central server is to be used. An introduction is expected in the near future.

In Singapore, a so-called contact tracing app (TraceTogether) was developed and made available by the Ministry of Health at a very early stage of the COVID-19 pandemic, which currently has about 1.1 million users (as of April 26, 2020, with a population of about 5.7 million). When the app is activated, a smartphone ID and the distance and duration of contact between different users are stored on the smartphone. If the test is positive, infected persons will be contacted by Singapore's Ministry of Health and must provide access to the stored IDs. The smartphone users are then notified as contacts. Bluetooth can only receive these contact details from other smartphones in close vicinity. Many other countries, including India, Israel and Austria, have now developed or are planning to develop or deploy similar apps.

Technical details and their implementation play a major role in questions of development and expected acceptance of such apps. On the one hand, it is a question of how much data is collected and stored, on the other hand, who has the data sovereignty, whether source codes are open and how securely the systems protect against attacks. A distinction is made between decentralized and centralized models. The approaches differ with regard to the use of central servers and the degree of control over the anonymous contact history by the app users. In a centralized model, the contact history is evaluated on the server; in the decentralized model, this is done by the app.

From the point of view of the Leibniz-ScienceCampus Digital Public Health Bremen (LSC DiPH), the following aspects should be given priority in the development and use of contact tracing apps in Europe:

General aspects:

• Contact tracing apps can be an important building block in the fight against a pandemic; however, they do not replace compliance with the necessary hygienic measures (distance, hand hygiene, disinfection etc.).
• Due to the special nature of the coronavirus, the pre-symptomatic phase is of great importance. Early contacting of contact persons in this phase through effective contact tracing has considerable potential to support the containment of the epidemic.
• It would not be helpful if a large number of different contact tracing apps were provided in parallel (exception: they are all interoperable). A solution is needed that is compatible with other approaches within Europe and ideally worldwide.

Acceptance, voluntariness:

• The use of a functioning app would have to be accepted by a large proportion of all smartphone users in order to achieve significant effects on the spread of the virus (probably at least 60-70% participation). Findings from health psychology and technology acceptance research should be used to achieve this goal.
• Voluntary use of the app is an essential basis for trust and acceptance; stigmatisation of non-users and an increasing digital divide must be prevented.
• In addition to providing the contact tracing app in various languages and considering principles of simple language, which is necessary to enable access and use for the broadest possible groups of the population, an accompanying intervention to increase use, considering principles of behavioral change, plays an important role - only when the app is used correctly it can fulfill its function.
• The notification of contact persons identified by the contact tracing app could not only take place when a positive test is present, but also in the case of justified suspicion. The prerequisite for this would be that both suspected cases and contact persons would be given access to the virus test as quickly as possible.
• In addition to contact tracing, further functions should be developed, e.g. to inform people about places with a high incidence of infections.

Data protection and anonymity:

• In principle, the stored ID data of the close contacts are to be stored primarily anonymously, i.e. it must not be possible to relate these IDs to individual users.