New animal research offers a compelling explanation as to how the omicron variant causes less severe disease than some of its predecessors: It seems to settle in the nose, throat, and windpipe, rather than traveling down to the lungs.

More than a dozen research groups have been observing the omicron variant in lab settings using animals this past month. More than six studies have shown omicron is milder than the delta variant and others. While previous variants have scarred the lungs and caused breathing difficulties in humans, several studies with mice and hamsters have shown that the variant caused much milder symptoms. This included research in Syrian hamsters, which had been found to become severely ill with other variants, The New York Times reported.

A large consortium of Japanese and American scientists released a report on their hamster and mice studies last week: Their findings included that the animals infected with omicron were less likely to die, in addition to losing less weight and having less lung damage.

Meanwhile, another study has gleaned more information about the virus using bits of tissue taken from human airways during surgery. University of Hong Kong, S.A.R., China researchers studied the 12 lung samples, finding that omicron grew more slowly than previous variants. While this could shed light on why people infected with omicron seem less likely to be hospitalized than those with delta, the question will need more research to verify the findings.

Ravindra Gupta, M.D., a virologist at the University of Cambridge, suggests a molecular explanation for why omicron does not appear to thrive in the lungs. The TMPRSS2 protein, carried by many cells in the lungs, does not grab onto omicron well. That means omicron does not effectively infect those cells as vigorously as delta, something that both Gupta's lab and a team from the University of Glasgow have independently discovered, The Times reported. Cells higher in the airway tend to not carry that protein and coronaviruses can also slip into cells that do not make the protein.

"It's all about what happens in the upper airway for it to transmit, right?" Gupta told The Times. "It's not really what happens down below in the lungs, where the severe disease stuff happens. So you can understand why the virus has evolved in this way."