Norfolk is likely to have up to 300 suspected cases of the Omicron, as the variant takes hold in the county.
Louise Smith, Norfolk County Council's director of public health, said the new Covid strain has increased significantly since last Friday, when there were just five confirmed cases and 12 suspected ones.
Official figures will be released on Friday (December 17) and Dr Smith expects that up to 50 county cases of Omicron will be confirmed, on top of the surge in suspected cases to around 300.
Already, daily cases of Covid - including all variants - are increasing sharply in Norfolk, more than doubling from about 400 to 850 over the last three days. It is not known how many of these are Omicron, as not all have been identified by laboratory testing.
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Dr Smith said that based on how cases caused by the variant are doubling each day, there could now be up to 300 suspected cases in the county.
Almost 12,000 Omicron cases have been confirmed nationally. It comes as another 88,376 confirmed coronavirus cases were reported in the UK - the second day in a row a new record daily number has been set. Again, it is not know how many of these are Omicron.
Dr Smith said: "I would say the pattern here is the same as the rest of the county. The only thing in our favour is we are starting from a lower base, as our rates had been quite low.
"We are estimating about half of our cases are now Omicron. These are mainly in working age adults at the moment, while it's still Delta in young children and school age children."
The number of people in hospital who have tested positive for Covid-19, has been relatively constant in the past few days - about 65 people.
Dr Smith said it was still too early to know whether Omicron would cause more severe disease, but it would be "overly optimistic" to think it would not lead to increased hospital admissions during January.
While she said people needed to "find a balance", she said: "Be particularly cautious where you or others are particularly vulnerable.
"It's clear that the more you come into contact with people, the higher the risk of getting coronavirus."
She urged people going to see more vulnerable people on Christmas Day to test themselves first.
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