More than 31,000 close contacts were identified during the first week of the test and trace system in England, figures show.
Out of 8,000 people testing positive for coronavirus, two-thirds provided details of who they had been close to.
Most were reached and provided the information within 24 hours, NHS figures show.
Around 25,000 contact tracers were recruited in England and started work at the end of May.
The figures, which cover 28 May to 3 June, show that 85% of contacts identified were reached and asked to self-isolate for 14 days.
What is test and trace?
It’s a way of controlling the spread of the virus by asking people who have tested positive for coronavirus to share information on who they have been in close contact with.
If it’s positive, tracers will text, email or call you and ask you to log on to the NHS Test and Trace website.
Close contacts will then be told to stay at home for 14 days, even if they don’t have symptoms.
This process is organised slightly differently around the UK.
What happens around the UK?
In Scotland, the system is called NHS Test and Protect, Between 28 May and 7 June 2020, 741 contacts were traced from 681 positive tests for the virus – an average of 1.5 contacts per case.
Northern Ireland was the first part of the UK to bring in contact tracing.
Contact tracing started in Wales on 1 June and is called ‘test, trace, protect’.