It has become the dominant version of the virus in two local authority areas already, new analysis shows.
The Omicron sub-variant, officially called BA.2, now accounts for a quarter of cases in England, up from one in 100 a month ago, according to research by the Wellcome Sanger Institute.
Stealth Omicron was named a ‘variant under investigation’ on January 21 and is now on track to become the dominant strain in the UK.
It got its nickname because it is more difficult to differentiate from Delta than the original Omicron variant, BA.1.
It also appears to be more contagious.
A spokesperson for the UK Health Security Agency (UKHSA) said its “apparent growth advantage is currently substantial”, when compared with Omicron.
Early studies show it takes around three days for Stealth Omicron to spread from an infected person to their contacts, half a day shorter than with Omicron.
The UKHSA believes this may account for its faster spread.
With coronavirus isolation rules in England set to end on Thursday, public health experts are still trying to find out whether BA.2 poses a greater threat than the original Omicron strain.
There is so far no detected change in the effectiveness of vaccines against symptomatic infection.
But there is not yet enough data to show whether BA.2 causes more severe disease than the original Omicron variant, the UKHSA has said.
The World Health Organisation said the BA.2 variant is now dominant in 10 countries worldwide, including China, India and Denmark.
It said: “Overall, there is no difference in severity when looking at countries where BA.2 is dominant and those where BA.1 is dominant.”
The Wellcome Sanger Institute analysed 38,000 positive Covid-19 tests taken in the week to February 12 to determine which variant they were.
Its research did not include Scotland, Northern Ireland or Wales.
Analysis of its findings shows there are two local authority areas - Woking and Northampton - where BA.2 is already the dominant strain.
Areas with fewer than 10 samples have been omitted.
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