Boris Johnson has said there is “some evidence” the new variant of coronavirus is more deadly than the old one.
Data suggests the variant, first identified in Kent, is associated with a 30% higher mortality rate.
His warning came as the government’s chief scientific adviser, Sir Patrick Vallance, said studies show the variant is susceptible to existing vaccines.
Speaking at a Downing Street press conference, the prime minister said: “We have been informed today that in addition to spreading more quickly it also now appears that there is some evidence that the new variant... may be associated with a higher degree of mortality.”
Outlining the evidence, Sir Patrick said that for a man in their 60s, the average risk was that for a thousand people who got infected, roughly 10 would be expected to die – whereas with the new variant it might be 13 or 14.
He stressed the evidence was not yet strong, and more research needs to be done.
Pressed on a particular paper about the new variant suggesting a 91% increase in mortality, Sir Patrick said: “Some of them suggested no increase in mortality at all... there’s also data from hospitals showing there’s no difference in outcome in patients in hospital”.
“There’s a lot of uncertainty, there’s a lot more work that needs to go on – I really urge against just picking the highest number and assuming that’s correct. I don’t think that’s the case at all.”
Less is known about the transmissibility of new variants from Brazil and South Africa, but there is concern they have features which could make them less susceptible to vaccines, Sir Patrick added.
The UK one appears to be between 30% and 70% more transmissible, but it is not currently understood why.
Fears that it was driving a surge in UK cases ultimately led to school closures and the introduction of Tier 4 measures in England, as well as Christmas visits being cancelled in many parts of the country.
In a sombre briefing from Number 10, the PM warned there was further hardship to come.
He struck a cautious note about any possible lifting of lockdown measures, saying that any easing would be gradual, and would start with the opening of schools.
Johnson said: “We really can’t begin to consider unlocking until we’re confident that the vaccination programme is working, until we’re confident that we don’t have new variants or changes in our understanding of the virus.
“I think the most important thing is we’ve got to be in a position where the rate of infection of the virus is not still so high, and it’s very high right now.
“So, for unlocking just to lead to another big rebound, I think that would be the wrong thing, the wrong way to approach this.”
The chief medical officer for England, Professor Chris Whitty, said there were signs coronavirus cases were falling – while hospitalisations in parts of England were beginning to “flatline”.
However, he said it will take weeks for death rate to start falling.
Watch: What is long COVID?