The COVID-19 variant identified in England last month could carry a higher risk of causing death although data is limited, according to one of the government's scientific advisory groups, ITV political editor Robert Peston said on Twitter on Friday.
Early evidence suggests the variant of coronavirus that emerged in the UK may be more deadly, Prime Minister Boris Johnson said.
"In addition to spreading more quickly, it also now appears that there is some evidence that the new variant - the variant that was first identified in London and the south east - may be associated with a higher degree of mortality," Johnson said.
"It's largely the impact of this new variant that means the NHS is under such intense pressure."
While the new variant is more contagious, Britain's chief health and science officials have so far said there was no evidence that it was more lethal or caused more serious illness.
However, Peston said the government's New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) had now concluded it "may be a bit more lethal than the existing strain."
He cited a statement from Neil Ferguson, an Imperial College professor and Nervtag member, as saying: "It is a realistic possibility that the new UK variant increases the risk of death, but there is considerable remaining uncertainty."
No one was immediately available for comment from Britain's Department of Health, which covers Nervtag.
The statement cited by Peston set out a 1.3-fold increased risk of death with the variant, but also highlighted that only some types of testing could specify which variant of the virus a patient had contracted.
"The big caveat is that we only know which strain people were infected with for about 8% of deaths," Peston quoted Ferguson as saying, also setting out further limitations on the available data.
Sir Patrick Vallance, the government's chief scientific advisor, told the BBC that the evidence on lethality "is not yet strong".
He said: "I want to stress that there's a lot of uncertainty around these numbers and we need more work to get a precise handle on it, but it obviously is a concern that this has an increase in mortality as well as an increase in transmissibility."
The new variant appears to be around 30% more deadly.
For example, with 1,000 60-year-olds infected with the old variant, 10 of them might be expected to die. But this rises to around 13 with the new variant.
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