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Background

Digital contact-tracing is being developed in several countries to tackle the SARS-CoV-2 pandemic. Manual contact tracing is too slow to reach people before they transmit, whereas the scalability and speed of a digital approach, using proximity sensors of smartphone devices, is theoretically fast enough to stop the epidemic​(Ferretti et al. 2020)​.

The development of an app includes a technological component and an epidemiological component. The technical component needs to ensure that the proximity events are recorded with sufficient precision in different circumstances and that protection of personal health-related data is ensured throughout the process. NHSX, the European PEPP-PT project (​ https://www. pepp-pt. org​), and the Norwegian FHI, are developing systems that are both functional and secure. Solving the technical aspect is necessary but not sufficient to secure its success. A functional contact tracing app that can successfully suppress the epidemic requires a transparent algorithm that is (1) epidemiologically sound,(2) has been assessed by simulation with extensive sensitivity analysis, and (3) can be audited and optimised as data from the app becomes available and the epidemic evolves.