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- Clinical microbiologist
Scientists have been "surprised" by properties of the Omicron variant as studies suggest coronavirus has "altered the way it infects cells", a professor has said.
Professor Ravi Gupta, a member of the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG), said experts were learning about the virus "all the time".
He warned the increase in deaths and cases of Omicron was a "serious concern".
Confirmed cases of the Omicron strain have risen by more than 12,000 in the UK - with London's cases alone topping 10,000 - according to the latest data on Sunday.
Prof Gupta told Sky News: "The sheer numbers that are going to be affected because of the increased transmissibility of this virus is going to potentially overwhelm our health service. So that's a really critical situation that we're facing."
He added: "Even if the vaccines protect us to a significant degree, then the increased transmissibility and penetration of the virus into communities that we're seeing already is putting a large amount of pressure (on the NHS) because a very small fraction of a very large number still translates to significant numbers being hospitalised."
Prof Gupta, professor of clinical microbiology at the Cambridge Institute for Therapeutic Immunology and Infectious Diseases, said Omicron has properties that have "surprised" scientists to some degree.
He said: "Some experiments that have been done in different groups are suggesting that the virus has kind of altered the way it infects cells.
"This is again potentially due to some mutations that have occurred, and there may, and this is a big caveat, there may be a slightly different profile for this virus [compared to Delta].
"Nonetheless, even if it were a little bit milder, in a very broad sense, the sheer numbers of cases would still translate into some individuals becoming quite sick.
"We're not anticipating that this virus, because of these changes, is going to suddenly become harmless. I think that shouldn't be the interpretation of the work that we and others are doing.
"But what we are saying is that we are learning about the virus all the time."
Prof Gupta said he is "in the middle viewpoint" about whether the government should take tougher measures to tackle the rising case numbers of the variant.
He said: "My personal viewpoint is that I'm in the middle of the two sort of views on this. I think the restrictions we've had were important.
"I think we could go further, but I'm not necessarily saying that a full lockdown is the essential policy decision, but, of course, I'm not the policy-maker and all I do is scientific research and advise accordingly."
In Britain, 12 people infected with Omicron have died, while 104 people with the variant are in hospital, according to latest figures from the UK Health Security Agency.
Earlier, Dominic Raab said he cannot make "hard, fast guarantees" that more restrictions won't be imposed in the run-up to Christmas.
However, the deputy prime minister said he believes Britons will have a "much better Christmas than last year".