London, Oct 24 : A sub-variant of the Covid Delta strain -- AY.4.2 -- may be up to 15 per cent more transmissible than the original Delta, scientists estimate.
AY.4.2 made up almost 10 per cent of all infections in England in the fortnight ending October 9, the Daily Mail reported.
Its prevalence in England doubled in a month from being behind four per cent of cases in September to 8.9 per cent in the two weeks to October 9, said scientists at the Wellcome Sanger Institute.
AY.4.2, which is one of 45 sub-lineages of Delta and dubbed by many as Delta plus, is likely to be named as Nu, the report said
The strain could be the most infectious subvariant seen since the pandemic began, Professor Francois Balloux, director of the University College London Genetics Institute, was quoted as saying to the Financial Times.
The World Health Organisation will likely elevate AY.4.2 to a "variant under investigation", which means it would be given a name under its Greek letter naming system, Balloux added.
However, he noted that since the UK is the only country where the sub-lineage has "taken off", its quick growth could also be a "chance demographic event".
"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," Balloux said.
"Here we are dealing with a potential small increase in transmissibility that would not have a comparable impact on the pandemic."
The UK has recorded about 50,000 new Covid infections, marking another three-month high.
Hospitalisations and deaths are also on the rise.
According to the latest weekly report from the UK Health Security Agency (UKHSA) -- which has replaced Public Health England -- revealed Covid cases have risen in all age groups, but are now at a record high among school children.
AY.4.2 has spread to all but two dozen places in the country.
Some experts have said the subvariant may be behind the surge in cases in the UK, which other European countries are not seeing to the same extent.
However, there is yet no evidence that AY.4.2 is more deadly than earlier versions of the Delta strain, which was first identified in India last December, the Daily Mail report said.
It includes two mutations -- called Y145H and A222V -- and is being monitored, the UKHSA said.
Both of these spike mutations have been found in other virus lineages since the pandemic began -- but are not present on any current variant of concern.