The Omicron variant would grow 70 times faster in the bronchi than the Delta

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In a context where cases of the Omicron variant of SARS-CoV-2 are increasing rapidly around the world, the World Health Organization (WHO) is concerned. A study by researchers in Hong Kong provides the first information on how this new variant infects the human respiratory tract. It is said to multiply 70 times faster than the Delta variant in the bronchi, but the Omicron infection in the lungs is said to be less than that of the original strain.

“Even if Omicron causes less severe symptoms, the number of cases could again overwhelm unprepared health systems,” warns WHO Director-General Tedros Adhanom Ghebreyesus. South Africa remains by far the most affected African country by the virus, especially since the emergence of Omicron. In fact, the country has registered the highest number of cases this last week with 122,870 contaminations, according to an AFP count based on official figures.

The study on the transmissibility of the Omicron variant was conducted by researchers at the LKS School of Medicine at the University of Hong Kong. It is currently under peer review for publication.

The Chinese team found that the Omicron variant infects and multiplies 70 times faster than the Delta variant of SARS-CoV-2 in human bronchi, which could explain why Omicron can be transmitted faster between humans than previous variants. Their study also showed that Omicron’s infection in the lungs is significantly less than that of the original strain, which could be an indicator of less disease severity.

Use of ex vivo respiratory tract cultures

To understand why the Omicron variant is different from others in terms of disease transmission and severity, the team of researchers used ex vivo cultures of the airways. Dr. Michael Chan Chi-wai (Principal Investigator at the Center for Immunology and Infection, Hong Kong Science and Technology Park) and Professor John Nicholls were the first to use it to study emerging viral infections since 2007, such as avian and eastern influenza. Medium respiratory syndrome coronavirus (MERS).

Specifically, this method uses lung tissue removed for treatment of the lung, normally discarded, to study viral diseases of the respiratory tract. Dr. Chan and his team successfully isolated the Omicron variant and used this experimental model to compare the infection of the 2020 SARS-CoV-2 (Alpha) strain, the Delta variant, and Omicron.

Omicron, on the other hand, affects the respiratory bronchi.

They then found that Omicron multiplied more rapidly in the bronchi than the original strain and the Delta variant. 24 hours after infection, the Omicron variant replicated approximately 70 times faster than the Delta variant and the Alpha strain. On the contrary, it replicated less efficiently (10 times less) in human lung tissue than the Alpha strain, which may suggest a lower severity of the disease. In short, the parent strain affects the lungs more, while Omicron affects the respiratory bronchi more.

© LKS College of Medicine, University of Hong Kong (HKUMed)

The 32 mutations of the Omicron variant have an impact on its transmissibility, but also on the immune system, as Dr. Chan recalls in a press release: “It is important to note that the severity of the disease in humans is not only determined by replication. of the virus, but also by the host’s immune response to infection, which can cause alteration of the innate immune system. It is also observed that by infecting more people, a highly infectious virus can cause more serious illness, although the virus itself is less pathogenic. So if we add to this our recent studies showing that the Omicron variant can partially evade the immunity conferred by previous vaccines and infections, the overall threat from Omicron is probably very significant. “

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