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Researchers in Hong Kong and the United Kingdom have independently found that the Omicron SARS-CoV-2 seems to proliferate in the upper bronchia rather than in the lungs, potentially leading to less severe disease.
The scientists found that Omicron infects and multiplies 70 times faster than the Delta variant and the original SARS-CoV-2 in the human bronchus, which also may explain why Omicron may transmit faster between humans than previous variants.
The research was conducted ex vivo at the HKLab at the University of Hong Kong and the Gupta Lab at Cambridge University.
“At 24 hours after infection, the Omicron variant replicated around 70 times higher than the Delta variant and the original SARS-CoV-2 virus. 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 Hong Kong study found.
“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, which may lead to dysregulation of the innate immune system, i.e. ‘cytokine storm’,” said Dr Michael Chan Chi-wai. “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.”
“Efficient infection of lung cells could correlate with severity of lung disease,” said Ravi Gupta, Professor of Clinical Microbiology Department of Medicine University of Cambridge. “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.”
“Omicron does appear to have become more immune evasive,” said Gupta, “but that properties associated with disease progression *may* be attenuated to some extent.”
The Gupta Lab also sought to determine how well antibodies from vaccinated individuals neutralised Omicron vs Delta. They found that Omicron was poorly neutralised after two doses of mRNA or Ad vectored vaccine compared to Delta, but that the third dose (mRNA vaccine) rescued this at an early time point.
