A new "subvariant" of the coronavirus Delta strain is spreading in the UK.
AY.4.2 is being monitored by scientists, who believe it may be slightly more transmissible than the strain first detected in India last year.
That means the Delta descendant could be the most infectious variant of the virus to date.
What is a subvariant? And how did this new one emerge?
All viruses change over time as they spread by acquiring random copying errors, or mutations.
Research in the UK by the Universities of Bath and Edinburgh found the virus that causes Covid-19 mutates about once a week, which is about 50 per cent higher than previous estimates.
Most mutations are harmless glitches that bring no benefit to the virus but sometimes they help, such as by enabling it to spread more easily.
This is how variants like Delta are created, by outcompeting slower-spreading strains.
Delta is estimated to be about 50 per cent more transmissible than the original virus detected in Wuhan, China.
Because it spreads faster, it replaced earlier variants to become the dominant SARS-CoV-2 strain around the world.
But it has continued to mutate, which has created a new descendant, or subvariant, of Delta.
How is the new Delta subvariant different?
It includes two mutations, Y145H and A222V, in a part of the virus called the spike, which it uses to latch on to cells.
The mutations have been seen before in other variants of the coronavirus.
“Most Sars-CoV-2 mutations have independently emerged many times in unrelated strains,” said Francois Balloux, director of the University College London Genetics Institute.
“Both the spike Y145H and A222V mutations have been found in various other SARS-CoV-2 lineages since the beginning of the pandemic, but have remained at low frequency until now.”
He said strains carrying both mutations were first detected in April 2020.
“The A222V was found in the B.1.177 lineage that swept Europe in the summer of 2020, but careful follow-up analyses pointed to the lineage likely having no inherent transmissibility advantage and that its spread was most likely caused by demographic processes,” Prof Balloux said.
Neither of the mutations are found in any variant of concern, which are strains considered inherently more severe or transmissible by the World Health Organisation.
And they are not obvious candidates for increased transmissibility, Prof Balloux said.
“But we have learnt that mutations can have different, sometimes unexpected, effects in different strains,” he said.
How common is the new strain in the UK?
It accounts for a growing but still small percentage of sequences in the UK, at about 6 per cent.
An National Health Service document published on Tuesday revealed the subvariant was one of four being monitored by scientists working for the UK Health Security Agency.
“A Delta sublineage newly designated as AY. 4.2 is noted to be expanding in England,” it said.
“It is now a signal in monitoring and assessment has commenced."
How common is it outside the UK?
It is rare. There have been only three cases detected in the US so far, Prof Balloux said.
In Denmark, which carries out regular genomic monitoring, the subvariant reached 2 per cent of sequenced cases but has faded since.
But if it is spreading in the UK, could the subvariant be more transmissible?
It could be. Prof Balloux said the recent rise in sequences of the descendant in the UK indicates it could be about 10 per cent more transmissible than the original Delta strain, so keeping an eye on it is “worthwhile”.
“The emergence of yet another more transmissible strain would be suboptimal. Though, this is not a situation comparable to the emergence of Alpha and Delta that were far more transmissible [50 per cent or more] than any strain in circulation at the time.
“Here we are dealing with a potential small increase in transmissibility that would not have a comparable impact on the pandemic.”
The sublineage will continued to be monitored in the UK and could be placed under investigation by the WHO.
Work is also under way to test whether it is less well recognised by antibodies, Prof Balloux said.