Chinese researchers have found evidence that the super-infectious Omicron variant may have emerged through chance encounters with mice. While their research has some caveats and limitations, they offer an interesting alternative to the prevailing theory that the variant evolved in a chronically ill person with a weakened immune system.
The research was published ahead of Christmas in the Journal of Genetics and Genomics, but a free draft of the article is available on pre-print server bioRxiv.
The SARS-CoV-2 Omicron variant was first reported on 24 November 2021 in South Africa. The variant was distinctly odd as it contained 45 mutations, some of which made it more resistant to vaccines and more contagious than other variants. One of its most interesting features is that many of these mutations had not been previously seen in other variants. In the evolutionary tree of SARS-CoV-2 variants, Omicron stands out as a particularly notable oddity.
To explain this oddity, a team from the Chinese Academy of Sciences in Beijing argues that this mutational sequence is different from viruses that evolve in human patients, but very similar to mutations associated with virus evolution in mouse cells. They also say the mutations indicate that the virus has adapted to infect mouse cells.
They say this suggests the virus may have jumped from humans to mice and accumulated these unusual mutations before jumping back to humans.
“Our results show that the ancestor of Omicron jumped from humans to mice, while rapidly accumulating mutations favorable to infecting this host then jumped back to humans, pointing to an interspecies evolutionary trajectory for the Omicron epidemic,” the study authors wrote in their paper.
However, this theory also has some question marks. First of all, SARS-CoV-2 is not very successful in infecting mice. The part of mouse cells that the virus will typically use to gain entry has very low affinity for the standard SARS-CoV-2 spike protein. Other researchers have succeeded in adapting SARS-CoV-2 to infect mice in the laboratory. This raises the question of whether lab animals are at the origin of the variant, but this question is not documented in the real world.
This makes it difficult, if not impossible, to understand how the virus spreads from humans to mice. Many scientists are skeptical of the Omicron’s animal origin story, but they also generally believe it’s too early to fully understand the variant’s origins. Still, this is certainly an important question as it could help us predict and prevent the rise of dangerous variants in the future.
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