The Omicron variant multiplies 70 times faster than Delta in the airways, according to a study.
Hong Kong University researchers also found the new variant replicates 10 times slower in the lungs than predecessors.
The finding may explain why the mutant virus is spreading at a ferocious pace, and also lends weight to the theory it is milder than the past variants, something which doctors in South Africa have been claiming for weeks.
The researchers exposed lung tissue in a laboratory to the original Covid strain that was identified in Wuhan last year, along with the two variants, to compare how the viruses behave after infection.
Omicron replicated faster in the bronchus — tubes connecting the windpipe and lungs — suggesting people with the strain may be more infectious.
Higher viral loads nearer the throat means people are more likely to breathe out viral particles.
Delta was found to duplicate much quicker in the lungs, where more of the virus can lead to the most severe illness.
The finding may be the biological clue behind why doctors insist people infected with the strain only suffer cold-like symptoms.
But Dr Michael Chan Chi-wai, a public health expert at Hong Kong University and chief investigator, warned speed of replication is only one way of measuring the severity of Covid infection and individuals can still become unwell with the virus.
And the threat from Omicron ‘is likely to be very significant’ and could lead to higher rates of severe illness and deaths at a population level than other strains by infecting many more people, he said.
The findings add to data, studies and reports from doctors on the ground that the virus spreads rapidly but causes less severe symptoms.
A real world study in South Africa found there was 29 per cent less severity from Omicron compared to earlier waves, while there were a third fewer hospitalisations.
It comes as the UK yesterday recorded its highest number of daily Covid cases since the pandemic began, with 78,610 confirmed.
The actual number infected will be many times higher, with the UK Health Security Agency estimating 200,000 people caught Covid on Monday alone.
However, the UKHSA has only reported 15 hospitalisations and one death linked with the variant.
At a Downing Street briefing last night, the Prime Minister urged Britons to scale back their Christmas plans and only socialise when necessary in a bid to control the spread.
Experts have warned that even if the virus is much milder, record numbers will become infected in the coming weeks which could lead to more hospitalisations than last winter.
The Hong Kong researchers used discarded lung tissue taken during operations to treat lung conditions.
Their results, which have not been published in full or peer-reviewed, show that 24 hours after becoming infected with Omicron, the virus replicated at a rate 70 times higher than Delta and the Wuhan strain in the bronchus.
But in the lungs, it replicated less efficiently – 10 times slower – than the original virus. This may explain why Omicron is thought to cause less severe disease.
Dr Chan 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.
‘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.’
Dr Muge Cevik, an infectious diseases expert at the University of St Andrews, said on Twitter: ‘For those asking whether this is good/bad news, simple answer: I don’t know.
‘While Omicron may infect the lung cells less efficiently, a higher viral load may worsen immune response.
‘The only thing I am sure of is that Omicron will spread so quickly through the population, making it likely impossible to contain even with the most stringent measures and giving us very little time over the next few weeks. So get your vaccines and boosters.’