COVID-19: New mutation of Delta variant under close watch in UK
Officials are keeping a close watch on a new descendant of the Delta variant of COVID that is causing a growing number of infections.
Delta is the UK's dominant variant, but the latest official data suggests that 6% of COVID cases that have been genetically sequenced are of a new type.
AY.4.2, which some are calling "Delta Plus", contains mutations that might give the virus survival advantages.
Tests are under way to understand how much of a threat it may pose.
Experts say it is unlikely to take off in a big way or escape current vaccines.
It is not yet considered a variant of concern, or a variant under investigation - the categories assigned to variants and the level of risk associated with them.
What is AY.4.2?
There are thousands of different types - or variants - of COVID circulating across the world. Viruses mutate all the time, so it is not surprising to see new versions emerge.
Original Delta was classified as a variant of concern in the UK in May 2021 after overtaking the Alpha variant to become the dominant type of COVID in circulation.
But in July 2021 experts identified AY.4.2.
This offshoot or sublineage of Delta has been increasing slowly since then. It includes some new mutations affecting the spike protein, which the virus uses to penetrate our cells.
So far, there is no indication that it is considerably more transmissible as a result of these changes, but it is something experts are studying.
The mutations - Y145H and A222V - have been found in various other coronavirus lineages since the beginning of the pandemic.