Up to 300 people who attended a charity football match are being told to self-isolate after 28 people tested positive for Covid-19.
The event at Burnside Working Men’s Club, in Fencehouses on the border of Sunderland and Durham, took place on 30 August.
Anyone who attended must self-isolate for 14 days from then.
Durham County Council has been working with Sunderland City Council and Public Health England to manage the response.
Durham director of public health Amanda Healy said contact tracing was being carried out to find anyone who had been in close contact with someone who tested positive.
She added: “However, we are also asking anyone who was at the charity event on Sunday 30 August to immediately self-isolate up until midnight on the 13th, unless they’re contacted individually by NHS Test and Trace with further advice.
“We are also asking people in the community who have already had a test which has come back negative to self-isolate for 14 days from the 30th because it can take up to 14 days for symptoms to appear.
“This is essential if we are to stop the spread of the virus in this community as quickly as possible.”
On Saturday another football match was called off in neighbouring South Tyneside, which has been added to England’s watch list as an “area of concern” over coronavirus.