Data from the Department for Health and Social care shows 10,879 people who tested positive for Covid-19 in Northamptonshire were transferred to the Test and Trace service between May 28 and November 18.
That means 1,718 new cases were transferred in the latest seven-day period.
Contact tracers ask new patients to give details for anyone they were in close contact with in the 48 hours before their symptoms started.
This led to 25,541 close contacts being identified over the period – those not managed by local health protection teams, which are dealt with through a call centre or online.
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But just 61.3% were reached – a figure that has fallen steadily over a five-week period.
Across England, 58.8% of contacts not managed by local health protection teams were reached and told to self-isolate by NHS Test and Trace in the latest week to November 18.
Local health protection teams deal with cases linked to settings such as hospitals, schools and prisons.
The contact tracing rate including these cases was 60.3% – up slightly from the week before when it was 60.7%.
Around 157,000 new cases were transferred nationally in the week to November 18, the highest weekly number since NHS Test and Trace was launched.