Cases of the Omicron sub-variant BA.2 have been in the UK for some time - the first example was found on December 6, 2021.
But numbers are growing both here and abroad, and the UK Health Security Agency (UKHSA) named it a ‘variant under investigation’ on January 21.
Early analysis suggests it has a faster growth rate than the Omicron strain currently dominant in the UK, BA.1, but there is not enough evidence to say whether it causes more severe illness.
Dr Meera Chand, COVID-19 Incident Director at the UKHSA, said: “So far, there is insufficient evidence to determine whether BA.2 causes more severe illness than Omicron BA.1, but data is limited and UKHSA continues to investigate.”
It is not known where Stealth Omicron originated, with the first cases found in the Philippines and high numbers being reported in Denmark.
It got its nickname because it is more difficult to differentiate from Delta than other Omicron variants.
The Wellcome Sanger Institute analysed 38,000 positive Covid-19 tests taken in the week to January 15 to determine which variant they were.
It found one in 100 samples were now Stealth Omicron across England but 12 local authority areas had rates of one in 20 or higher.
Here are the 12 areas which had the highest percentage of the strain.