The UK Health and Security agency is investigating a subvariant of Omicron Covid, with 53 cases found so far in the UK.
The subvariant, which is reported to be outpacing the original mutant strain in some parts of the world, has been designated Covid BA.2.
It has been dubbed Stealth Omicron by some scientists as it cannot be distinguished from older variants by standards PCR tests.
The World Health Organization (WHO) reports that Omicron, also known as B.1.1.529, has three main substrains, BA.1, BA.2, and BA.3
The UKHSA posted on Twitter: "The number of BA.2 cases is currently low, with the original Omicron lineage, BA.1, still dominant in the UK and further analyses will now be undertaken."
A spokesman said: "As of January 10 2022, 53 sequences of the BA.2 sub-lineage of Omicron had been identified in the United Kingdom.
"This sub-lineage, which was designated by Pangolin on 6 December 2021, does not have the spike gene deletion at 69-70 that causes S-gene target failure (SGTF), which has previously been used as a proxy to detect cases of Omicron. UKHSA are continuing to monitor data on the BA.2 sub-lineage closely."
It is believed the Stealth strain may be outcompeting BA.1 in Denmark - but scientists say the rates of infection, disease and hospitalisation are the same for both subvariants.
Stealth Omicron has gone from 20% to 45% of cases in Denmark, and there are 30,000 new cases a day in the country.
There are reports it is starting to spread quickly in the UK, Norway, Sweden, France and India.
Denmark’s Statens Serum Institut said: “Initial analysis shows no differences in hospitalisations for BA.2 compared to BA.1. It is expected that vaccines also have an effect against severe illness upon BA.2 infection.”
Researchers believe there are 28 new mutations between BA.1 and Stealth Covid BA.2.
Mutations are what can alter the behaviour of a strain - making it potentially more infectious, or altering how ill it makes sufferers.
