Scientists are still unable to confirm why the Omicron variant is highly transmissible yet less infective than the original virus or other variants, but early evidence from animal experiments suggests Omicron may not infect cells deep in the lung as easily as those in the upper airways.
Scientists turned to animal studies because it is challenging to judge whether Omicron intrinsically causes milder disease or because it's affected by vaccination levels since the number of people who have gained immune protection against COVID-19 through vaccination, infection, or both has grown over time.
Experiments in mice and hamsters demonstrated at least ten times lower number of viruses in their lungs infected with Omicron compared to those infected with the prior variants. Other experiments using lung cells identified a potential player in this disparity – a protein called TMPRSS2, which protrudes from the surfaces of many lung cells but is notably absent from the surfaces of most nose and throat cells. Since it cannot gain easy access to lung cells, it potentially lingers in the upper airways, maintaining very high concentration and making it easier for the virus to find a newer host and infect them.
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