Data from the Department for Health and Social care shows 1,286 people who tested positive for Covid-19 in Northumberland were transferred to the tracing system between May 28 and September 30 – generating 3,381 ‘close contacts’ - but only 69.2% of those were reached.
That means 1,040 people were not contacted or did not respond.
The failed traces were slightly down from the 70.2% reached in the period to September 23 – while positive cases increased by 393 over the week.
Test and Trace asks these patients to give details of anyone they were in close contact with in the 48 hours before their symptoms started and attempts are them made to trace them.
The figures come as the number of people testing positive for the coronavirus in England increased by more than half in one week, with health chiefs warning "prompt action" is needed to prevent a tide of hospitalisations.
In England, 51,500 people tested positive for Covid-19 in the seven days to September 30.
That was the highest total since the Test and Trace regime was launched in May – but the figures show that only 67% (34,500) were transferred into the contact tracing system.
Chris Hopson, chief executive of NHS Providers, said the large increase in positive tests, alongside worsening test and trace performance nationally, is "worrying."
He said: “We need prompt action to prevent a full-blown second Covid surge.”