Photo/Illutration The brown color shows hamster cells infected with the Omicron variant. The picture on the left, taken a day after the infection, shows the infected cells around a bronchiole. The other picture, taken three days after infection, shows the Omicron variant did not spread to within the lung tissues. (Provided by Kei Sato, associate professor of virology at University of Tokyo’s Institute of Medical Science)

The Omicron variant of the novel coronavirus appears less likely to cause serious symptoms because it stops at the “entrance” of lung tissues, a Japanese research team experimenting on hamsters has found.

Animal tests have shown that the previously dominant Delta variant caused more serious cases of pneumonia than Omicron, which is driving the sixth wave of novel coronavirus infections in Japan.

But the difference between the two strains’ ability to cause serious illness has not been made clear.

The research team, known as G2P-Japan and comprising scientists from the Institute of Medical Science of the University of Tokyo, Hokkaido University and other institutions, found that hamsters infected with the Delta variant showed symptoms, such as weight loss or impaired breathing functions, while hamsters infected with the Omicron strain exhibited milder symptoms.

The team collected samples of lung tissues from the hamsters and dyed brown only the cells infected with the novel coronavirus.

One day after infection of either the Omicron or Delta variants, the cells located around bronchioles—the airways that deliver air to the lung tissues--were brown.

Therefore, the brown color represented cells that are the “entrance” to the lung tissues.

Three days after the infection, the hamsters carrying the Delta variant showed brown cells within the lung tissues, meaning the virus had spread inside the organs.

However, the brown color remained only at the “entrance” of lung tissues in the hamsters infected with the Omicron variant, unchanged from the day after they were first infected with the strain, the researchers said.

The study, published in the science journal Nature on Feb. 1 at (https://doi.org/10.1038/s41586-022-04462-1), noted that the researchers cannot be sure if the reaction in the hamsters would be the same in humans.

But they said the findings suggest that the Omicron’s inability to pierce the lung tissue is one reason behind its reduced risk of hospitalization compared with the Delta strain.

Studies have shown that the novel coronavirus  causes infected cells to fuse to neighboring cells.

However, the G2P-Japan team found that this ability to fuse to neighboring cells decreases for cells infected with the Omicron variant. The team previously reported that the Delta variant is more likely to cause the fusion, which heightens its pathogenicity.

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The brown color shows hamster cells infected with the Delta variant. The picture on the left, taken one day after the infection, shows infected cells around a bronchiole. The other picture, taken three days after the infection, shows the Delta variant has spread to the inside of lung tissues. (Provided by Kei Sato, associate professor of virology at University of Tokyo’s Institute of Medical Science)

“Although details are still unknown, perhaps the Delta variant drives the fusion of cells, enables the destruction of cells around the entrance to lung tissues, which acts like a ‘barrier,’ and spreads the infection inside the lung tissues,” said Kei Sato, associate professor of virology at the Institute of Medical Science.

“By contrast, the Omicron variant doesn’t drive cell fusion as much and cannot cause the destruction of cells around the entrance to lung tissues,” he said. “This is probably why the Omicron variant only causes mild illness in many cases.”

Sato said this mechanism could also help explain why the Omicron variant is more contagious than Delta.

If the Omicron variant remains stuck in cells around bronchioles, it will be unable to enter the lung tissues and will instead travel through the bronchioles and be discharged to the outside world, he said.

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