Why is Omicron less dangerous than Delta?

Animal studies explain why intensive care hospitalizations are much lower, with fewer acute respiratory distress syndromes, when infected with the Omicron variant.

While the Omicron variant is causing alarm because of its very high transmissibility (three times that of Delta), it is hitting less hard, according to currently available data, especially from South Africa and the United Kingdom. So far, the explosion of the pandemic has not resulted in an overall increase in the number of deaths.

Although cases of COVID-19 have increased worldwide, hospitalizations have increased only slightly and Omicron appears to cause less severe illness than previous strains. The risk of being hospitalized is reportedly 60% to 70% lower than with the Delta variant, according to a study conducted by the British health agency UKHSA. British doctors are finding that their Omicron patients need mechanical ventilation less often and have shorter hospital stays.

“A growing body of new data suggests that the Omicron variant of the coronavirus responsible for COVID-19 causes less severe symptoms than previous strains,” a World Health Organization (WHO) official said on January 4.

Omicron is less dangerous,” said Olivier Véran. It causes less acute respiratory distress syndromes, the need for resuscitation beds is less than with previous variants.”

Omicron does not go into the lungs

The reason? First of all, the fact that Omicron infects people who have already been vaccinated – and are therefore better protected against severe forms – explains this lesser severity. But above all, several studies carried out by teams of researchers show that the Omicron variant multiplies less well in the lung cells. In other words, it causes less damage to the lungs than previous strains of the coronavirus.

“We have more and more studies suggesting that Omicron infects the upper respiratory tract, unlike other strains, which could cause severe pneumonia,” explained Dr. Abdi Mahamud, head of a team working on COVID-19 pandemic management at the UN organization, while stressing that this could be “good news.

Two studies conducted on mice and hamsters, reported at the end of December by The New York Times show that the Omicron variant attacks the upper respiratory tract: the nose, throat, and trachea. Unlike the initial strains, Alpha or Delta, which also infected the lungs where they could trigger the famous cytokine storm, a particularly strong inflammatory reaction.

The report, published on December 29 by Japanese and American scientists, shows that animals infected with Omicron have fewer lung lesions, lose less weight, and are less likely to die. Experiments conducted on hamsters – animals usually severely ill when infected with the coronavirus – show surprising results: their symptoms are much milder than with previous strains of the virus.

“This was surprising because all the other variants strongly infected these hamsters,” Dr. Michael Diamond, a virologist at the University of Washington and co-author of the study, told the NYT.

The question of strong contagion remains 

If these studies bring hope, they remain to be confirmed. Moreover, it does not explain why Omicron is so contagious. Sara Cherry, Ph.D., a virologist at the University of Pennsylvania, tells the NYT that it could be related to the greater presence of virus in the saliva and nasal passages.

In any case, even if less dangerous, this variant could pose a real threat to countries where vaccination coverage against COVID-19 remains low, the WHO said on Tuesday, January 4. Catherine Smallwood, a WHO emergency manager, explained that soaring infection rates could have the opposite effect. “The more Omicron spreads, the more it is transmitted and the more it replicates, the more likely it is to generate a new variant,” she said.


  • OMS
  • Studies Suggest Why Omicron Is Less Severe: It Spares the Lungs, New York Times, 31 décembre 2021.

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