Photo/Illutration An electron microscope image of the Omicron variant of the novel coronavirus (Provided by the National Institute of Infectious Diseases)

Scientists have found that the Omicron variant causes less severe illness in animals than other variants of the novel coronavirus, such as the Delta strain.

The study was conducted by a group of researchers from the Institute of Medical Science at the University of Tokyo, the National Institute of Infectious Diseases and other institutions to better understand the Omicron variant.

The findings were published in the science journal Nature on Jan. 21.

Omicron was first reported in November 2021 and quickly spread across the world, raising concerns over whether it can evade current countermeasures.

It is significantly more transmissible than past variants, and some have suggested the risk of developing severe illness from contracting it is lower than with Delta and other variants. But for the most part, its characteristics have remained a mystery.

The researchers infected hamsters with Omicron and other variants to study and compare how the viruses grow and what kind of symptoms the animals develop.

The rodents lost weight after several days once they became infected with the Delta variant. But those infected with the Omicron variant did not change in weight.

The amount of virus in the rodents’ lungs was comparatively small with Omicron, the researchers said. On the other hand, they did not find a significant difference in the amount of virus in the hamsters’ nasal passages.

Yoshihiro Kawaoka, a special appointment professor at the Institute of Medical Science at the University of Tokyo, said it remains unclear why the Omicron variant is more infectious even if the amount of virus in the nose does not increase.

“I don’t know the reason at this point," he said. "It is something that should be determined in the future.”

The researchers studied the hamsters’ lungs using computerized tomography.

Infected with the Delta variant, the hamsters developed inflammation in their lungs similar to that of COVID-19 patients.

But infected with the Omicron variant, the researchers only observed mild inflammation.

With the Delta variant, lung function declined. But with the Omicron variant, lung function was almost the same as with uninfected hamsters.

The researchers said the study shows the likeliness of the virus to multiply in the lungs and its ability to cause disease are both much lower with the Omicron variant.

But they cautioned it is still unknown whether the results found in rodents can be applied to humans and added that further research is needed.