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Did Omicron Come From Mice? Chinese Scientists Believe So


Tom Hale

Tom is a writer in London with a Master's degree in Journalism whose editorial work covers anything from health and the environment to technology and archaeology.

Senior Journalist

Lab mouse.

In the evolutionary tree of SARS-CoV-2 variants, Omicron sticks out like a sore thumb. Image credit: Billion Photos/

Researchers in China have gathered evidence that the super infectious Omicron variant may have originated through a chance encounter with mice. Their work has some caveats, but it provides an interesting alternative to the prevailing theory that the variant evolved in a chronically sick person with a weak immune system. 

Their study was published in the Journal of Genetics and Genomics on Christmas Eve, but a free-to-read draft of the paper can be found on the pre-pint server bioRxiv


The SARS-CoV-2 Omicron variant was first reported in South Africa on November 24, 2021. The variant is distinctly strange as it contains 45 mutations, some of which appear to make it more resistant to vaccines and more infectious compared to other variants. Most curiously, many of these mutations have not been seen before in other variants. In the evolutionary tree of SARS-CoV-2 variants, Omicron sticks out like a sore thumb.

To explain this oddity, a team from the Chinese Academy of Sciences in Beijing argues that this array of mutations differs from the viruses that evolved in human patients, but closely resembled the mutations associated with virus evolution in mouse cells. Furthermore, they say that the mutations show that the virus has adapted to infecting mouse cells. 

This, they believe, suggests that the virus may have hopped over to mice from humans where it accumulated these unusual mutations before jumping back into humans. 

“Our results suggest that the progenitor of Omicron jumped from humans to mice, rapidly accumulated mutations conducive to infecting that host, then jumped back into humans, indicating an inter-species evolutionary trajectory for the Omicron outbreak,” the study authors write in their paper. 


However, there are some drawbacks to this theory. Firstly, SARS-CoV-2 isn’t great at infecting mice. The part of mouse cells which the virus would typically use to gain entry has little affinity for the standard SARS-CoV-2 spike protein. Other researchers have managed to adapt SARS-CoV-2 to infect mice in the lab – raising the question of whether lab animals are involved in its origins – but it’s not been documented in the real world. 

This makes it difficult, albeit not impossible, to understand how the virus made this leap from humans into mice. Many scientists have been skeptical of the animal-origin story of Omicron, although generally, they believe it’s too early to fully understand the origins of the variant. Nevertheless, it’s an important question as it could help us predict and prevent the rise of dangerous variants in the future.


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