Via GenomeWeb, this article in the New York Times [archived copy: https://archive.fo/eJuTe ] is effectively a write-up in lay language of a slew of recent research papers. It is important enough that I am sharing most of it with you.
In studies on mice and hamsters, Omicron produced less damaging infections, often limited largely to the upper airway: the nose, throat and windpipe. The variant did much less harm to the lungs, where previous variants would often cause scarring and serious breathing difficulty.“It’s fair to say that the idea of a disease that manifests itself primarily in the upper respiratory system is emerging,” said Roland Eils, a computational biologist at the Berlin Institute of Health, who has studied how coronaviruses infect the airway. […]
But as cases skyrocketed, hospitalizations increased only modestly. Early studies of patients suggested that Omicron was less likely to cause severe illness than other variants, especially in vaccinated people. Still, those findings came with a lot of caveats.For one thing, the bulk of early Omicron infections were in young people, who are less likely to get seriously ill with all versions of the virus. And many of those early cases were happening in people with some immunity from previous infections or vaccines. It was unclear whether Omicron would also prove less severe in an unvaccinated older person, for example.Experiments on animals can help clear up these ambiguities, because scientists can test Omicron on identical animals living in identical conditions. More than half a dozen experiments made public in recent days all pointed to the same conclusion: Omicron is milder than Delta and other earlier versions of the virus.On Wednesday, a large consortium of Japanese and American scientists released a report on hamsters and mice that had been infected with either Omicron or one of several earlier variants. Those infected with Omicron had less lung damage, lost less weight and were less likely to die, the study found.Although the animals infected with Omicron on average experienced much milder symptoms, the scientists were particularly struck by the results in Syrian hamsters, a species known to get severely ill with all previous versions of the virus.“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.
[…]
The reason that Omicron is milder may be a matter of anatomy. Dr. Diamond and his colleagues found that the level of Omicron in the noses of the hamsters was the same as in animals infected with an earlier form of the coronavirus. But Omicron levels in the lungs were one-tenth or less of the level of other variants.
A similar finding came from researchers at the University of Hong Kong who studied bits of tissue taken from human airways during surgery. In 12 lung samples, the researchers found that Omicron grew more slowly than Delta and other variants did.The researchers also infected tissue from the bronchi, the tubes in the upper chest that deliver air from the windpipe to the lungs. And inside of those bronchial cells, in the first two days after an infection, Omicron grew faster than Delta or the original coronavirus did.These findings will have to be followed up with further studies, such as experiments with monkeys or examination of the airways of people infected with Omicron. If the results hold up to scrutiny, they might explain why people infected with Omicron seem less likely to be hospitalized than those with Delta.Coronavirus infections start in the nose or possibly the mouth and spread down the throat. Mild infections don’t get much further than that. But when the coronavirus reaches the lungs, it can do serious damage.
Immune cells in the lungs can overreact, killing off not just infected cells but uninfected ones. They can produce runaway inflammation, scarring the lung’s delicate walls. What’s more, the viruses can escape from the damaged lungs into the bloodstream, triggering clots and ravaging other organs.Dr. Gupta suspects that his team’s new data give a molecular explanation for why Omicron doesn’t fare so well in the lungs.Many cells in the lung carry a protein called TMPRSS2 on their surface that can inadvertently help passing viruses gain entry to the cell. But Dr. Gupta’s team found that this protein doesn’t grab on to Omicron very well. As a result, Omicron does a worse job of infecting cells in this manner than Delta does. A team at the University of Glasgow independently came to the same conclusion.Through an alternative route, coronaviruses can also slip into cells that don’t make TMPRSS2. Higher in the airway, cells tend not to carry the protein, which might explain the evidence that Omicron is found there more often than the lungs.Dr. Gupta speculated that Omicron evolved into an upper-airway specialist, thriving in the throat and nose. If that’s true, the virus might have a better chance of getting expelled in tiny drops into the surrounding air and encountering new hosts.“It’s all about what happens in the upper airway for it to transmit, right?” he said. “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.”
Read the whole thing, as they say. One of many interesting graphs from this preprint:
https://doi.org/10.21203/rs.3.rs-1211792/v1 Michael Diamond et al., “The SARS-CoV-2 B.1.1.529 Omicron virus causes attenuated infection and disease in mice and hamsters”
On a tangentially related note, it seems that the milder, more transmissible nature of omicron is causing even Anthony Faux-Xi to backtrack from long-held positions: the WSJ’s “Best of the Web” sarcastically describes him discovering that there is such a thing as an economy and that lockdowns and extended isolation mandates gravely impact it.
But is it all the different nature of omicron changing the calculus? Or, as Instapundit and others suspect, is a cynical pivot taking place for purely political reasons, because the (anti)Democratic party fear an electoral bloodbath in the November midterm elections, and are counting on the short-term memory of the “kiesvee” [Dutch: “electoral cattle”, freely: “sheeple”]?
I personally suspect it is a combination of both.
ADDENDUM: Great new video by Dr. John Campbell on the same research quoted in (1).
Spin, strangeness, and charm 
[…] have basically flatlined — there is even a hint of a decrease. This is consistent with the emerging understanding that omicron is almost exclusively an upper respiratory system disease, unlike the older variants […]