The Omicron sub-variant, officially called BA.2, is now dominant in Leeds, analysis shows.
BA.2 accounted for 57 per cent of cases in Leeds in the last week of February, according to research by the Wellcome Sanger Institute.
The strain also accounted for 57 per cent of all cases across England, up from a quarter just two weeks before.
It comes amid a recent rise in both cases and hospitalisations.
Stealth Omicron got its nickname because it is more difficult to differentiate from Delta than the original Omicron variant, BA.1.
It is also more contagious but early studies suggest it carries no greater risk of hospitalisation.
Dr Jenny Harries, chief executive of the UK Health Security Agency (UKHSA), said case numbers were lower than at the peak of the Omicron wave.
She said: “However, the increasing presence of the BA.2 sub-lineage of Omicron and the recent slight increase in infections in those over 55 show that the pandemic is not over and that we can expect to see Covid circulating at high levels.”
Professor Paul Elliott, director of Imperial College London’s React programme, said England is also seeing a rise in hospitalisations and warned that the BA.2 variant needs to be tracked carefully.
He added: “It is more transmissible. We are seeing an uptick in infections, particularly in the older group, and we are seeing an uptick in hospitalisations.
“At the moment, we’re possibly seeing the beginning of an uptick, but we don’t know where it’s going to go.”
The Wellcome Sanger Institute analysed 27,000 positive Covid-19 tests taken in the week to February 26 to determine which variant they were.
Its research did not include Scotland, Northern Ireland or Wales.
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