The omicron variant causes less damage to the lungs than previous strains of the coronavirus, according to a study conducted on hamsters and mice and released Wednesday by U.S. and Japanese scientists in In Review.

Rodents infected with omicron had less lung damage, lost less weight and were less likely to die. Infections were often limited to the nose, throat and windpipe.

''This was surprising, since every other variant has robustly infected these hamsters,'' said Dr. Michael Diamond, a virologist at Washington University and a co-author of the study.

Omicron levels in the lungs of the hamsters were one-tenth or less of the level that resulted from the other variants, whereas the level of the strain in the noses of the animals was the same as animals infected with an earlier strain, according to the study.

''Moreover, the attenuation in hamsters also was surprising, given that all prior SARS-CoV-2 variants have replicated relatively equivalently and to high levels in this animal,'' the researchers wrote.

The U.S. reported 2.49 million coronavirus cases in one week, a record high.

But early evidence shows that omicron, while highly contagious, is less virulent.

''We know now, incontrovertibly, that this is a highly, highly transmissible virus. We know that from the numbers we're seeing,'' Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told reporters in a White House briefing Wednesday.

But ''all indications point to a lesser severity of omicron versus delta,'' he said. 

Omicron is now the dominant strain of coronavirus, but delta remains prominent.

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