Omicron multiplies 70 times faster than Delta in airways, says Hong Kong study

The Omicron variant of coronavirus multiplies 70 times faster than Delta in human airways, according to researchers who said their work provides the first information on how it infects people.

But the study, from Hong Kong, also suggested that Omicron 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 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.

Reacting to the university’s release on the study, Dr David Strain, senior clinical lecturer at the University of Exeter, said it is unclear how the results of this laboratory-based work will compare to what is seen in patients.

He said: “The 70-fold increase in replication is of concern which does account for the increased transmissibility of the virus.

“There is a lack of clarity however, as to how this 10-fold reduction in lung infectivity in this lab-based study will translate in patients.

“At first glance it looks good news, however if the virus can replicate 70 times quicker, but infects 10 times slower, that still results in a seven-fold increased risk of disease (70 times more virus, even with a 10-fold reduction in ability to enter the cells will cause the virus to overwhelm the cells).”

Dr Julian Tang, honorary associate professor and clinical virologist in respiratory sciences at the University of Leicester, said the research teams at the university in Hong Kong are “very experienced in investigating emerging respiratory viruses”.

He added: “This new finding supports what we are starting to see epidemiologically – with rapid spread but milder disease – mostly mediated by upper rather than lower respiratory tract infections.”

He said it is in keeping with how a new virus tends to evolve “killing less, but infecting more” and that it remains to be seen if the pattern detected by the researchers is “repeated across populations, globally”.