The Omicron variant may be less efficient at attacking the lungs, researchers have found - but they warned it represents a "major public health challenge".
A Cambridge Institute of Therapeutic Immunology and Infectious Disease study suggested that the mutant strain could reduce how it replicates in the lungs and causes severe disease.
Further work is needed to find out if this is the case, the lead scientist said.
But the sheer number of cases poses a huge threat to the population, experts say.
Omicron is ripping through the UK, with more than half a million people testing positive in the past week.
Earlier today Health Secretary Sajid Javid refused to rule out restrictions with the number of cases rising at an alarming rate.
The study's abstract said: "These observations highlight that Omicron has gained immune evasion properties whilst compromising on properties associated with replication and pathogenicity."
Sage member Ravi Gupta, who headed the study, called on people not to be too reassured by the latest findings.
Prof Gupta, professor of clinical microbiology at Cambridge, tweeted: "What does this all mean? Efficient infection of lung cells could correlate with severity of lung disease.
"Syncitia or fused cells are often seen in respiratory tissues taken following severe disease. Delta was very good at both, in contrast to Omicron. Further work is needed.
"In summary this work suggests that Omicron does appear to have become more immune evasive, but that properties associated with disease progression may be attenuated to some extent. The significant growth of Omicron nevertheless represents a major public health challenge."
It comes after advisory group Sage's "situation update" said it is "almost certain that there are now hundreds of thousands of new Omicron infections per day" in England, prompting reports that tighter pandemic restrictions could be introduced after Christmas.
Last week a report by a team from the University of Hong Kong suggested the variant grew 10 times slower in lung tissue - which could be a sign of lower disease severity.
Michael Chan Chi-wai said: “It is also noted that by infecting many more people, a very infectious virus may cause more severe disease and death even though the virus itself may be less pathogenic.
“Therefore, taken together with our recent studies showing that the Omicron variant can partially escape immunity from vaccines and past infection, the overall threat from the Omicron variant is likely to be very significant.”
Earlier today Health Secretary Sajid Javid did not rule out new coronavirus measures before Christmas.
Asked on The Andrew Marr Show on BBC One whether he could rule it out, the Health Secretary said: "We are assessing the situation, it's very fast-moving. We've seen with Omicron there's a lot that we still don't know about Omicron.
"That's the truth of the matter. The reality is there's a lot of uncertainty."
And he said it was "time to be more cautious".
"We know this thing is spreading rapidly. We know already now that in London, it's around 80% of infections, in England it's around 60 per cent of infections," he added.
