How Omicron spreads so fast is the question of the moment. New research is pointing to the answer

New research from a team in Hong Kong offers a clue to why the Omicron variant is spreading so astonishingly fast around the world: it may be multiplying 70 times quicker than earlier strains within our lower airways.

Hong Kong study offers clues on transmission but questions remain over disease severity, symptom presentation

An enlarged view of human bronchus tissue infected with the Omicron variant of the SARS-CoV-2 coronavirus (in red) in laboratory research from a team in Hong Kong. (Supplied by the LKS Faculty of Medicine at The University of Hong Kong)

New research from a team in Hong Kong offers a clue to why the Omicron coronavirus variant is spreading so astonishingly fast around the world: it may be multiplying 70 times quicker than earlier strains within our lower airways.

The laboratory-based study, led by researchers from the LKS Faculty of Medicine at the University of Hong Kong, was shared online as a press release on Wednesday and is currently undergoing peer review for publication.

The researchers found that, just 24 hours after infection, Omicron multiplies 70 times faster than either the Delta variant or the original SARS-CoV-2 virus within tissue samples of human bronchi — the two large tubes that carry air from your windpipe to your lungs.

It's a finding that could explain why Omicron seems to transmit faster between humans than previous variants, and offers a striking contrast to how the variant replicated in the research team's samples of actual lung tissue compared to the bronchi. Within those lung samples, the variant multiplied at a rate more than 10 times slower than the original virus.

It's the lungs — not the bronchi — that are linked to potentially life-threatening COVID-19 complications such as pneumonia and, in severe cases, acute respiratory distress syndrome or ARDS. So if these findings hold up in a real-world setting, the team suspects that slower replication in the lungs might mean reduced severity of disease.

The finding also echoes early evidence from South Africa suggesting Omicron may be linked to a milder course of illness.

But severity of disease is not only a function of where and how quickly a virus replicates in a certain organ, "it is also a function of our immune response," said Dr. Dominik Mertz, an infectious diseases specialist with Hamilton's McMaster University, in an email exchange with CBC News.

People need to be careful in drawing direct conclusions from lab-based studies compared to how infections will actually present in the real world, he wrote.

"The latter must be based on observations in those who have confirmed infections, and we still have to learn more in terms of actual severity in a population like ours which has first and foremost mRNA-vaccine immunity."

Omicron cases have been identified in more than 70 countries around the world, including Canada, where the variant is expected to take over. (Evan Mitsui/CBC)

Omicron threat likely 'very significant'

The lead researcher from Hong Kong — Dr. Michael Chan Chi-wai, an associate professor of the university's school of public health — also acknowledged that disease severity is tied to other factors, like whether someone's immune system goes haywire in response to a coronavirus infection in what's known as a "cytokine storm."

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"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," he said in a statement. 

"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."

Ryan Troyer, a virologist and researcher at Western University in London, Ont., stressed that the Hong Kong team's results are still preliminary.

"The differences they're finding here look fairly significant," he said. "It'll be interesting to see if this is backed up by additional studies."

Electron micrograph of the human bronchus tissues after infection with SARS-CoV-2. Red arrows show viral particles. (Supplied by the LKS Faculty of Medicine at The University of Hong Kong.)

Variety of symptoms, from mild to severe

Troyer also noted that the study adds to the growing pile of evidence that COVID-19 infections may present in a variety of ways, even before the rise of the Omicron variant, with symptoms ranging from severe and life-threatening to far milder and perhaps barely-noticeable.

Take the ongoing testing of professional sports teams like the NBA and NHL, for instance. Troyer said those tests often pick up infections among the young, healthy players that the athletes themselves didn't even notice — since vaccination status, the age of those infected, and other factors all play a role in disease severity. 

A Tuesday presentation from Discovery Health, South Africa's largest private health insurance administrator, offered an analysis of an Omicron outbreak using more than 200,000 COVID-19 test results.

The team noted that for patients with Omicron who needed acute care outside of a hospital setting, the incubation period is typically only three to four days, with patients recovering a few days after that — a tighter timeframe than the previous estimated incubation period ranging from one to 14 days.

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"The most common early symptom seems to be a scratchy or sore throat," said Dr. Ryan Noach, CEO of Discovery Health, during the presentation.

Nasal congestion and dry cough are also commonly reported, he continued, along with muscle aches or pain manifesting in different areas. "Particularly in lower back pain, which seems to be a hallmark in the out-of-hospital presentation," he said.

Such run-of-the-mill symptoms are easy to disregard, but experts say that's one of the many ways a more-transmissible variant might be spreading — both quickly and quietly.

"I think it's important for people to think about the fact that you can't assume that lack of symptoms means lack of virus," Troyer said.


Lauren Pelley

Senior Health & Medical Reporter

Lauren Pelley covers health and medical science for CBC News, including the global spread of infectious diseases, Canadian health policy, and pandemic preparedness. Her 2020 investigation into COVID-19 infections among health-care workers won best in-depth series at the RNAO Media Awards. Contact her at:

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