NHS COVID-19 app

The app was developed by NHSX, the digital innovation unit of the NHS, in collaboration with the UK's National Cyber Security Centre and tech companies such as Google and Apple. Initially, the UK government pursued a centralized model, but due to privacy concerns and technical limitations, it switched to the decentralized Apple/Google Exposure Notification API in June 2020. The app used Bluetooth Low Energy to measure signal attenuation as a proxy for distance, and it assigned random rotating IDs to users to protect privacy. Data was stored locally on devices, and only the app's backend received minimal information when a user reported a positive test.¹

Key features included contact tracing, venue check-in via QR codes, a symptom checker, and local outbreak alerts. Users could report their own symptoms or a positive test result, triggering anonymous alerts to other users who had been in close proximity for at least 15 minutes at a distance of under 2 meters. The app also provided a countdown timer for self-isolation and links to testing services. By early 2021, the app had been downloaded over 21 million times, and it was estimated to have prevented hundreds of thousands of infections.

Studies suggested the app was effective in reducing transmission, with one analysis estimating that for every 1% increase in app users, the infection rate dropped by 0.8%–2.3%. However, the app faced criticism over its limited interoperability with other countries' apps, and its reliance on smartphone ownership raised concerns about digital exclusion. Additionally, there were early technical issues, such as battery drain and false alerts, which were later addressed.

One lesser-known feature was the 'risk score' calculation, which used a complex algorithm that considered both distance and duration of exposure, and was later updated to include infectiousness of the index case. The app also had a 'check-in' feature that allowed users to see if they had visited a venue where an outbreak had been reported. In Wales, the app was integrated with the NHS Wales Test, Trace, Protect service. The app's code was made open source on GitHub, allowing independent security audits.

Exposure Notification API

A framework developed by Apple and Google to enable Bluetooth-based contact tracing while preserving user privacy.

Bluetooth Low Energy

A wireless technology used for short-range communication, employed by the app to detect nearby devices.

QR code

A matrix barcode that can be scanned to log a user's presence at a venue.