Omicron symptoms: Why they may be milder than other variants

Toronto -

There are new clues as to why the Omicron variant may produce milder COVID-19 symptoms, despite being more transmissible.

Several recent studies that looked at the effect of the virus on mice and hamsters have found that the Omicron variant leads to a lower viral load in the lungs and less damage to the tissues compared to previous variants of SARS-CoV-2.

All of these rodent studies are pre-prints, which means they have yet to be peer-reviewed or published in a medical journal.

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One study from Belgian researchers published online on Boxing Day looked at how Omicron infected Syrian hamsters. The researchers found that the hamsters with Omicron had a 99.9 per cent lower viral load in the lungs compared to hamsters infected with older strains of the virus.

Syrian hamsters have been considered an important tool for COVID-19 research, as these animals are known to be highly susceptible to getting seriously ill from the virus.

A similar study looking at Syrian hamsters and led by British researchers was also released on Dec. 26. The researchers compared 11 hamsters infected with the Omicron variant to six infected with previous variants, including Delta.

They found that Omicron-infected hamsters had fewer signs of weight loss, ruffled fur and breathing difficulties -- all of which are clinical signs of COVID-19 in Syrian hamsters.

"Further investigation is required to conclusively determine whether Omicron is less pathogenic in Syrian hamsters and whether this is predictive of pathogenicity in humans," the authors wrote.

A third study that looked at both mice and Syrian hamsters was published by U.S. and Japanese researchers on Dec. 29. Once again, the researchers found that rodents with Omicron had less weight loss and a lower viral load in their respiratory tracts.

The Omicron-infected rodents also had less damage to the lung tissues, suggesting that the variant can't replicate as well in the lungs.

Another U.K.-led study published on Dec. 30 looked at the effect of Omicron on mice. Much like the studies that looked at hamsters, the researchers found that the mice infected with Omicron lost less weight and had better signs of recovery compared to mice that had the Delta variant and other older strains.

Oral swabs were also taken from all of the mice two, four and six days after infection. The viral loads in the lower and upper respiratory tract were found to be 100-fold lower in the mice with Omicron.

"Our results, and emerging from human observational studies, suggest that the Omicron variant may lead to less severe and/or more rapid recovery from clinical disease reflected in reduced hospitalization," the authors wrote.

However, the researchers warned that the high transmissibility of the new variant could still inundate hospitals, despite evidence of milder symptoms.

"The higher transmissibility could still place huge burden upon health care systems even if a lower proportion of infected patients are hospitalized," they wrote.

 

FINDINGS SIMILAR IN HUMANS, BUT OMICRON STILL A THREAT

All of these studies found that Omicron leads to a lower viral load in the lungs. Dr. Nelson Lee, who is the interim director of the Institute for Pandemics at the University of Toronto's Dalla Lana School of Public Health, says those findings are consistent with what's been observed in the lungs of human Omicron patients.

"All these experiments show that there is reduced infectivity of the virus to lung cells, basically showing a low virus level. That is also reflected in the clinical manifestation," Lee, who was not involved in any of the papers, told CTVNews.ca over the phone on Monday.

A pre-print Hong Kong study from mid-December also found that while Omicron can multiply 70 times faster in airways, the variant replicates itself 10 times slower within lung tissues.

McGill University infectious disease specialist Dr. Donald Vinh notes that the advantage of animal studies is the fact that researchers can closely examine how the virus affects the lungs and other organs, something that can't be done with humans until autopsy studies can be conducted.

"You can intentionally infect (the rodents) and then you can actually cut them open and look at their organs, which obviously, we don't want to do in humans," Vinh told CTVNews.ca during a phone interview on Monday.

Vinh says the rodent studies show that Omicron "could be non-severe in the right context" However, he believes the results should be taken with a grain a salt, given that these studies don’t focus on humans.

"(There's) some encouraging data at both the epidemiological and now in the rodent models that suggest Omicron could have the potential to cause disease less severe than Delta, for example. But it's not the same thing as saying that it's benign," he said.

Lee also warned that these results of the rodent studies "cannot be overgeneralized to humans."

"We should be very cautious when we interpret this data," he said.

"There's still a lot of people that develop severe illnesses because of this Omicron infection … The low risk doesn't mean it will never happen."