Explained: How Omicron dodges immune system

Unlike other Covid variants, Omicron appears to be able to evade the antibodies generated by all other variants, according to a study.

Explained: How Omicron dodges immune system
Geneva: Unlike other Covid variants, Omicron appears to be able to evade the antibodies generated by all other variants, according to a study.

A team from the Centre for Emerging Viral Diseases of the University of Geneva (UNIGE) and of the Geneva University Hospital (HUG) analysed the antibody neutralisation capacity of 120 people infected with the original SARS-CoV-2 strain, or with one of its Alpha, Beta, Gamma, Delta, Zeta or Omicron (sub-variant BA.1) variants.

In vaccinated individuals, while the neutralisation capacity is also reduced, it remains far superior to natural immunity alone. This could explain why Omicron is responsible for a net increase in vaccine break-through infections, but not in hospitalisations, revealed the study published in the journal Nature Communications.

"Omicron proved to be the most effective at evading pre-existing natural immunity, as well as, to a lesser extent, that induced by vaccination," said Benjamin Meyer, researcher at the UNIGE.

Indeed, antibody levels against ancestral SARS-CoV-2 in vaccinated people are roughly 10 times higher than in people who have only developed post-infection immunity.

Moreover, the combination of the two, known as hybrid immunity, seems to maintain even higher and broader reactive antibody levels.

"Thus, Omicron can evade existing immunity and cause an infection, but hospitalisation and death due to Covid-19, even with Omicron, is still reduced after vaccination."

Nevertheless, SARS-CoV-2 retains an astonishing ability to mutate, which also appears to be accelerating. "Vigilance is still required, especially as the epidemiological curves have been rising sharply since the appearance of BA.5, the most recent Omicron sub-variant," said Isabella Eckerle, Professor at UNIGE.

The scientists also performed antigenic mapping of the different variants, based on the same model used to determine the antigenic changes of influenza.

This computational method makes it possible to represent the distance between the characteristics of the antigens of the variants concerned: the further away they are, the less effective the antibodies generated during previous infections will be.

It turns out that the Omicron serotype is totally different from the others, as was the Zeta variant, which disappeared rapidly, said Meriem Bekliz, a postdoctoral researcher in Eckerle's lab.

"This virus, which belongs to the same family of coronaviruses that cause almost a third of common colds, can, in the absence of measures, spread like wildfire in populations with no immunity to fight it," Meyer said.

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