Covid-19: why Omicron seems less severe than the Delta variant

Study after study, scientists seem to confirm the impression that the Omicron variant causes less severe forms of Covid-19 than the Delta strain.

Since December 23, there have even been four publications going in the same direction: greater transmissibility of the virus, but less dangerous than Delta.

Researchers at the University of Liverpool concluded, in an article published online on December 26, that mice infected with the Omicron variant lost less weight and had less severe forms of pneumonia than mice subjected to other mutations. of Sars-CoV-2.

Golden hamsters, used by scientists at the University of Louvain (Belgium), had a similar reaction to rodents in Liverpool, in the results also published on December 26 of another experiment to assess the dangerousness of the variant detected in November 2021.

Away from the lower respiratory tract

Work by American researchers submitted a few days before the end of the year to the scientific journal Nature and that the Guardian was able to consult confirm that mice contaminated with the Omicron variant have less serious forms of Covid-19. Finally, the British data at the end of December “show that a person affected by the Omicron variant seems to have between 50% and 70% less risk of being hospitalized”, underlines Ian Jones, virologist at the University of Reading, contacted by France 24.

"We can therefore say that from an epidemiological point of view, it is a variant which is less severe than the previous one", concludes Björn Meyers, virologist at the Faculty of Medicine of the University of Magdeburg, joined by France 24. “Not only do there appear to be fewer hospitalizations, but people who are admitted also stay for shorter periods and with milder symptoms,” adds Lawrence Young, virologist and professor of molecular oncology at Warwick Medical School, contacted. by France 24.

The new scientific studies also suggest that this apparent lesser severity of Omicron may have to do with how the virus infects cells in the human body.

“He uses the same door to enter the cell but without using the same key,” Lawrence Young sums up metaphorically.

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Omicron still clings to the cell with its famous Spike protein, but the many mutations in this part of the virus mean that it prefers to multiply in the nose, throat and bronchi, without venturing as much towards the lungs. as was the case with previous strains of Sars-CoV-2.

This essential difference allows it to be more resistant to vaccines, while being potentially less dangerous because “in all respiratory diseases, the health risk is greater when the virus attacks the lower respiratory tract, such as the lungs.

This is what happens with the H5N1 avian influenza agents that lodge in the lower respiratory tract, ”explains Ian Jones of the University of Reading. 

Beginning of the endemic phase?

“This is probably why we are dealing with a virus that causes less severe forms of the disease while being more transmissible since there is a higher concentration of the virus in the nose and throat, which increases the risk. contamination ”, summarizes Jonathan Stoye, virologist and researcher at the Francis Crick Institute in London, contacted by France 24.

But for this expert, one should not overestimate the drop in dangerousness of Omicron either.

“We see that it multiplies more easily and faster in the upper respiratory tract than previous strains, and there will always be a proportion of pathogens that will eventually find their way to the lungs.

This is where vaccination is important because it limits the risk of contracting serious forms of Covid-19 ”, specifies this specialist. 

“While it is true that it seems to be less 'severe' than the Delta variant, Omicron still appears to be more dangerous than the Alpha variant [dominant in part of Europe at the end of 2020, Editor's note] of which it is a more direct descendant ”, specifies, meanwhile, Björn Meyer, the virologist of the University of Magdeburg.

So there are two ways of looking at Omicron.

On the one hand, it is a more serious form than its ancestor - Alpha - but on the other, it appears more benign than the Delta variant that it is supplanting globally.

And from a health point of view, it is this last aspect that matters to Ian Jones, because “it is perhaps with this variant that we entered the endemic phase of the epidemic [ that is to say that the disease takes root, but in forms more harmless to health, Editor's note]. ”

“In any case, it would be in line with what we know about the history of coronaviruses,” recognizes Jonathan Stoye, the head of research at the Francis Crick Institute in London.

Some precedents - like the Russian flu pandemic of 1889 - suggest that these viruses tend to start to subside after about two years. 

Here again, “it makes sense from an evolutionary point of view, since the variants which will prevail are those which are the most contagious, that is to say those which are rather present in the upper respiratory tract, where they are generally less dangerous for health ”, summarizes Ian Jones. Clearly, a kind of viral Darwinism could give a boost to the vaccine effort to overcome the pandemic.

In the meantime, Omicron's meteoric spread is putting great strain on the healthcare system.

Because even if the variant is less virulent, hospitals must "manage a strong absenteeism on a daily basis [of the nursing staff] linked to the rapid progression of contaminations with the arrival of Omicron", explains to France 24 Gilles Pialoux, head of department infectious and tropical diseases at Tenon Hospital (AP-HP).

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