The government is keeping a “close eye” on a mutation of the Delta coronavirus variant thought to be up to 10% more transmissible amid a rise in UK cases.
AY.4.2 was discovered by scientists tracking the evolution of the Delta variant, which is now dominant after reaching Britain from India earlier this year.
AY.4.2 is “noted to be expanding in England”, the UK Health Security Agency (UKHSA) reports, and now makes up around 6% of all UK cases, according to the latest data from the end of September.
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On Monday, figures showed 49,156 people had tested positive for coronavirus in the previous 24-hour period, the largest number since July, and raising fears for the winter ahead.
The average number of cases over the past seven days was 16% higher than the previous week.
The prime minister’s official spokesman said: “[AY.4.2] is something we’re keeping a very close eye on.
“There’s no evidence to suggest that this variant… the AY.4.2 one… is more easily spread.
“There’s no evidence for that, but as you would expect, we’re monitoring it closely and won’t hesitate to take action if necessary.”
Variant offshoots are common and form when the virus makes mistakes as it copies itself to replicate.
Delta already has 45 such offshoots. The most common of which, AY.4, now accounts for 70% of all SARS-CoV-2 genomes sequenced worldwide.
AY.4 has given rise to two recognised offshoots, AY.4.1 and AY.4.2.
Francois Balloux, director of the University College London Genetics Institute, said while AY.4.2 is “likely to be up to 10% more transmissible”, it’s not comparable to the original explosion of Delta – which was at least 50% more infectious than Alpha when it arrived in Britain.
He said the infectiousness of AY.4.2 isn’t enough to explain the UK’s rise in cases, adding: “Here we are dealing with a potential small increase in transmissibility that would not have a comparable impact on the pandemic.”
AY.4.2 is still largely confined to the UK and not yet classed as a variant of concern, although it is being monitored by UKHSA and “is likely to be elevated to the rank of ‘variant under investigation'”, according to Professor Balloux.
According to UKHSA, AY.4.2, includes spike mutations A222V and Y145H.
Professor Balloux said these mutations were not “obvious candidates for immune escape, increased transmissibility or higher virulence”.
He added: “As AY.4.2 is still at fairly low frequency, a 10% increase in its transmissibility could have caused only a small number of additional cases.
“As such, it hasn’t been driving the recent increase in case numbers in the UK.”
Reports in the US have suggested concern about AY.4.2, particularly with international travel between the UK and the States due to resume from November.
AY.4.2 cases remain rare outside the UK. There have been only three cases detected in the US so far.
In Denmark, cases reached a 2% frequency, but this has since decreased.
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Scott Gottlieb, former US Food and Drug Administration commissioner, suggested AY.4.2 made up 8% of all cases sequenced in the UK.
He tweeted on Sunday: “UK reported its biggest one-day Covid case increase in 3 months just as the new delta variant AY.4 with the S:Y145H mutation in the spike reaches 8% of UK sequenced cases.
“We need urgent research to figure out if this delta plus is more transmissible, has partial immune evasion? The variant has been in the UK since about July, but it has been slowly increasing in prevalence.
“There’s no clear indication that it’s considerably more transmissible, but we should work to more quickly characterize these and other new variants. We have the tools.”
Dr Meera Chand, COVID-19 Incident Director at the UK Health Security Agency, said: “We continue to monitor and investigate genomic changes in SARS-COV-2 and to take the appropriate public health actions.”
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