If an app user tests positive for coronavirus, they can choose to share their result anonymously. The NHS will then send alerts to other app users who have been in ‘close contact', over the last few days. These alerts will never identify an individual.
‘Close contact’ is based on an algorithm, but generally means you've been within 2 metres of someone with coronavirus for 15 minutes or more. Read more about how the risk scoring algorithm works.
If the app user who tested positive booked their test through the app, the test result will come through to their app automatically. However they still need to click ‘share random IDs’ before their close contacts can be notified. If they booked their test through another route, they will also need to link their test result into the app using a code.
This means that you will not always receive a close contact alert on the same day that the person received their positive test result. Your self-isolation period is calculated from the date at which you were in close contact with the person who tested positive. Read more about how self-isolation countdown timer is calculated.