The Omicron variant of Covid multiplies 70 times faster than Delta in human airways, say scientists.
But researchers said the variant replicates less well in human lung tissue compared with the original strain of the virus, which they said could indicate a lower severity of disease.
The Hong Kong study's lead professor warned that the "overall threat from Omicron variant is likely to be very significant".
Dr Michael Chan Chi-wai said it is important to note "that the severity of disease in humans is not determined only by virus replication but also by the host immune response to the infection".
The associate professor, at the school of public health at LKS Faculty of Medicine at The University of Hong Kong (HKUMed), added: "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 Omicron variant is likely to be very significant."
A release, published by HKUMed, stated that researchers had successfully isolated the Omicron variant and used lung tissue removed for treatment of the lung, to investigate the new mutation.
They compared Omicron with the original strain, and with Delta.
The release said they found that Omicron "replicates faster than the original Sars-CoV-2 virus and Delta variant in the human bronchus".
They said that 24 hours after infection, the Omicron variant "replicated around 70 times higher than the Delta variant and the original Sars-CoV-2 virus".
They added: "In contrast, the Omicron variant replicated less efficiently (more than 10 times lower) in the human lung tissue than the original Sars-CoV-2 virus, which may suggest lower severity of disease."
The research is currently under peer review for publication, the university said.
It comes after Covid cases rose in 60 per cent of areas in the UK in a week - with areas of London seeing an explosion in infections driven by Omicron.
England's chief medical officer went on to warn that records would continue to be set in coming weeks after 78,610 cases were reported in 24 hours - the highest of the pandemic so far.
