The NHS COVID-19 app uses Bluetooth Low Energy (BLE) to understand the distance, over time, between app users. If someone tests positive for coronavirus (COVID-19), the app’s risk-scoring algorithm uses this data, along with the infectiousness of the individual testing positive, to make calculations about risk and work out whether a user should get an alert.
We've published the risk-scoring algorithm that summarises how the app will initially calculate a risk score for someone based on their recent contact with people who report that they have tested positive for coronavirus.
The algorithm has been optimised by scientific experts, based on the functionality provided by V1.6 of Google's and V2.0 of Apple's Application Programming Interface (API).