The app uses an algorithm to work out who gets an alert when someone they’ve been near tests positive for coronavirus.

The algorithm uses anonymous data based on Bluetooth signal strength to make calculations about risk. This is based on the physical distance between app users and how much time they have spent near each other. Bluetooth signal strength is reduced through walls and therefore less likely to satisfy the necessary criteria.  

NHS doctors and scientists are continuously updating this algorithm to make it as accurate as possible. Read more about how the risk-scoring algorithm works.