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People exit Piccadilly Circus underground station, in London, on Oct. 19.Alberto Pezzali/The Associated Press

Scientists in Britain are monitoring a new version of the Delta variant, called Delta Plus, that has begun to spread in England and could be slightly more contagious.

The new variation, officially designated as AY. 4.2, accounted for 6 per cent of all COVID-19 cases that were genetically sequenced during the week of Sept. 27, according to a recent report from the U.K. Health Security Agency. The agency added that the mutation was “on an increasing trajectory.”

The virus that causes COVID-19 has mutated thousands of times since the pandemic began and most versions have either petered out or had little impact. The AY. 4.2 mutation was first detected in Britain in July and it has been growing slowly. Scientists have yet to designate it a “variant under investigation” or a “variant of concern,” which is an indication of the level of risk. If it is elevated to a “variant under investigation,” which is widely expected, the World Health Organization will assign it a Greek letter under its naming system.

Delta was classified as a variant of concern last May in Britain after it began to overtake the Alpha variant, which emerged in late 2020.

Scientists said AY. 4.2 has two key mutations in the virus’s spike protein that helps it enter human cells. These mutations have emerged before, but the number of cases has remained low until now. “Neither mutation is a priori an obvious candidate for increased viral transmissibility, but we have learnt that mutations can have different, sometimes unexpected, effects in different strains,” said Francois Balloux, a professor of computational systems biology at University College London. He added that AY. 4.2′s two spike protein mutations have not been found in other variants of concern.

Britain is a world leader in tracking genetic changes to the virus and so far the AY. 4.2 hasn’t been found in many other countries. In Denmark, which also has an extensive genomic surveillance program, the new variation accounted for 2 per cent of sequenced cases but the number has fallen lately.

Britain has seen a surge in COVID-19 cases in recent weeks, which some have feared could be linked to the new mutation. The number of new infections topped 49,000 on Monday, an increase of 20 per cent in one week and the highest daily figure in three months. The tally fell back to 43,738 on Tuesday.

“We need urgent research to figure out if this Delta Plus is more transmissible, has partial immune evasion,” Scott Gottlieb, the former head of the U.S. Food and Drug Administration, said on Twitter on Sunday after noting the rising number of cases in Britain. “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. Balloux and other experts have suggested that the AY. 4.2 variation could be between 10 per cent and 15 per cent more transmissible than the Delta variant. However, Dr. Balloux said it likely didn’t cause the recent jump in British infections. “As AY. 4.2 is still at fairly low frequency, a 10-per-cent increase in transmissibility could have caused only a small number of additional cases,” he said.

“The emergence of yet another more transmissible strain would be suboptimal,” he added. “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.”

Other scientists have said Britain’s rising case load is connected to low vaccination rates among children and inadequate measures to slow transmission in schools. Britain was slow to launch a vaccination program for children, and face coverings are not compulsory in schools across most of England.

“The rate of infections among secondary-school-aged children is clearly the driving force behind this sustained tide of new infections,” said Simon Clarke, an associate professor of cellular microbiology at the University of Reading. “With infection rates going up, and little appetite for new restrictions on movement or activities, vaccines appear to be our best hope of avoiding more trouble ahead. With so much COVID-19 in the population, we must hope that the jabs continue to protect most people against serious disease.”

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