Nearly 200 people in localities stretching from Silverstone to Brackley tested positive over seven days to December 3.
Seven-day case rates in Brackley North soared to 1,506 and in Silverstone, Syresham and Helmdon to 1,434.5 per 100,000 people, according to official figures. The rate across all of England is 493.3.
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Around 13,400 people live in the two areas.
Local health officials say the rise in infections is down to community transmission and advised people county-wide to heed government guidance over face masks and working from home even before the rules become enforceable.
UK Health Security Agency is leading an investigation into 28 Omicron, which had been identified in the Brackley area by Wednesday — more than double the number revealed on Saturday — with another 55 cases said to be 'highly probable.'
Lucy Wightman, director of Public Health Northamptonshire said: “It is now clear that the variant has spread within the community, mainly through a series of events and within households.
"Currently known Omicron cases have only been detected in a reasonably young age range and to date no one locally has been hospitalised.
"However we are still learning about Omicron and the related risks. So the sensible option is to keep calm, exercise caution, and follow the new Government Plan B rules announced on Wednesday. "
Under new rules announced by Prime Minister Boris Johnson last night:
■ From Friday, face masks will be required in more public settings — including theatres and cinemas
■ From Monday, people will be asked to work from home where possible
■ From Wednesday, the NHS Covid Pass will also be required for visitors to nightclubs, indoor unseated venues with more than 500 people, unseated outdoor venues with more than 4,000 people and any event with more than 10,000 people
Mr Johnson said Christmas parties and nativity plays should still go ahead — as long as the guidance is followed.
He said: "The best way to ensure we all have a Christmas as close to normal as possible is to get on with Plan B."
"Irritating though it may be, it is not a lockdown."
■ Book a PCR test HERE■ Book a vaccine jab HERE■ Order lateral flow test kits HEREA mobile PCR testing unit has been set up a Brackley Leisure Centre for anyone with symptoms or close contacts of Covid-19 cases, although tests MUST be booked online HERE or by the 119 phone service.
Around 4,500 people positive tests were recorded in Northamptonshire during the seven days to yesterday, according to government figures — up by around 50 percent from last month.
Anyone with coronavirus symptoms — a continuous cough, high temperature or loss or change in taste or smell — should book a PCR test immediately and self-isolate until seeing a negative result.
Those without symptoms who could still be carrying the virus are advised to take a lateral flow test twice weekly at home.
Mrs Wightman added: “Testing is a really important tool in our armoury.
"Around one in three people with Covid-19 do not have symptoms and these rapid lateral flow tests — which can be picked up from local pharmacies ordered online — help find people who are still infectious and can give the virus to others.
"This is why it’s also important to take a test before going out and mixing with people in crowded indoor places, or visiting someone who is at higher risk of getting seriously ill from Covid-19."
“Regular and thorough hand-washing remains an essential part of good hygiene and prevents the spread of many viruses, colds, flu and stomach bugs.
“And finally, vaccination provides powerful protection against severe illness so getting the jab is still the most important thing you can do to protect yourself."