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New Delta descendant may be more infectious than its ancestor

Scientists are anxiously tracking a descendant of the Delta coronavirus, which is responsible for a growing proportion of Covid-19 cases in the UK, and could be more infectious than the original Delta variant, they say.

This AY.4.2 subvariant has only recently been recognised by virologists who follow the genetic evolution of Delta but it already accounts for almost 10 per cent of UK cases. Its prevalence is increasing rapidly, though not as fast as the original Delta variant when it reached Britain from India early this year.

Two experts — Jeffrey Barrett, director of the Covid-19 Genomics Initiative at the Wellcome Sanger Institute in Cambridge, and Francois Balloux, director of the University College London Genetics Institute — said AY.4.2 seemed to be 10 to 15 per cent more transmissible than the original Delta variant, which has come to dominate Covid cases around the world.

If the preliminary evidence is confirmed, AY.4.2 may be the most infectious coronavirus strain since the pandemic started, said Balloux. “But we have to be careful at this stage,” he added. “Britain is the only country in which it has taken off in this way and I still would not rule out its growth being a chance demographic event.”

AY.4.2 “is likely to be elevated to the rank of ‘Variant under Investigation’,” Balloux said, at which point the World Health Organization would assign it a Greek letter under its naming system.

Some commentators in the US linked the emergence of AY.4.2 with the very high levels of Covid cases, hospitalisations and deaths in the UK — far above those recorded elsewhere in western Europe.

The UK on Monday announced a daily total of 49,156 people testing positive for Covid, the largest number since July. The average over the past seven days was 16 per cent higher than the previous week.

“We will obviously keep a close watch on cases,” said the prime minister’s spokesperson. “We always knew the coming months would be challenging . . . The vaccination programme will continue to be our first line of defence, along with new treatments, testing and public health advice.

Scott Gottlieb, former US Food and Drug Administration commissioner, tweeted on Sunday: “We need urgent research to figure out if this ‘delta plus’ is more transmissible, has partial immune evasion.”

UK experts said research was already under way. Barrett added: “While it [AY.4.2] may make things more difficult, it doesn’t by itself explain the recent high UK caseload.”

Coronavirus is more prevalent in Britain mainly because of reduced emphasis on measures such as mask-wearing, social distancing, ventilation and working from home than in neighbouring countries.

Use of the phrase “Delta plus” has caused much confusion and should be avoided, Barrett added. Commentators have already applied the term to previous descendants of Delta with mutations different from those on AY.4.2.

AY.4.2 is one of 45 sub-lineages descending from Delta that have been recorded around the world. It carries two characteristic mutations in the spike protein with which the virus infects human cells, called Y145H and A222V.

Both mutations have been recorded individually in previous coronavirus lineages, though not in ‘Variants of Concern’, said Balloux, and it is not clear why they might make Delta more infectious. “They are not obvious candidates for immune escape, increased transmissibility or higher virulence,” he said.

Ravi Gupta, professor of clinical microbiology at Cambridge university, said it was hard to predict the effect of individual mutations on viral behaviour, because they affect the shape and behaviour of the spike protein in a way that scientists do not yet understand. “We don’t really know how Delta itself is achieving such high infectivity,” he said.

The Alpha variant that spread rapidly through the UK last year was about 50 per cent more transmissible than the original virus from Wuhan, and Delta added a further 60 per cent or so on top of Alpha. AY.4.2 will not add transmissibility on anything like that scale, Gupta said.


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