Omicron has now pushed Delta entirely out of many parts of England, research shows.
The Wellcome Sanger Institute analysed more than 34,000 positive Covid-19 tests taken in the week to 1 January to determine which variant they were.
In more than 30 local authority areas, Omicron was the only variant found.
The number of Delta samples found across England has fallen dramatically in recent weeks - suggesting that Omicron cases are replacing rather than adding to those caused by Delta.
Here, we look at the percentage of Covid-19 infections which are estimated to be Omicron, according to Wellcome Sanger Institute data for the week to 1 January 2022.
What is the World Health Organisation saying?
Dr Hans Henri Kluge, the WHO’s regional director for Europe, said the highly contagious variant had become “a new west-to-east tidal wave sweeping across the region, on top of the Delta surge that all countries were managing until late 2021”.
He said: “It is quickly becoming the dominant virus in Western Europe and it is now spreading into the Balkans.
“At this rate, the Institute for Health Metrics and Evaluation forecasts that more than 50% of the population in the region will be infected with Omicron in the next six to eight weeks.”
But Dr Kluge said death rates “remain stable and continue to be highest in countries with high Covid-19 incidence combined with lower vaccination uptake”.