More than one in four people who test positive cannot be reached by the test and trace system in England, data shows.
In the first two weeks of operation to 10 June, the service was given the details of 14,000 people to follow up.
But fewer than 10,200 could be reached. Some failed to provide the right contact details, while others did not return calls.
The service has asked over 87,000 of their close contacts to self-isolate.
That represents nine in 10 of the people they have contacted.
What is not clear – because the government is not publishing these figures – is how quickly close contacts are being reached after the positive case is tested.
To be most effective, this should be done within 48 hours of the test being carried out.
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