Omicron variant may have originated in mice, study reveals

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BEIJING, China — Scientists are still uncertain how the original COVID-19 strain came to be, but new research indicates the Omicron variant may have originated in mice before making the jump to humans.

Emerging in late 2021, the Omicron COVID-19 variant has further complicated and extended an already exhausting pandemic. Much more infectious than earlier versions of SARS-CoV-2, Omicron now accounts for the vast majority of U.S. infections. It didn’t take long for the scientific community to establish that Omicron emerged from a strain that was circulating in mid-2020. Besides that, though, researchers have largely been unsuccessful at retracing the variant’s viral steps as it evolved into its current form.

One popular working theory among scientists is that animals carried Omicron before eventually infecting humans through further mutations. This latest study, conducted by Jianguo Xu from the National Institute for Communicable Diseases Control and Prevention China CDC, supports this idea, concluding the most likely carriers are mice. To be clear, study authors caution more research is necessary before they can confirm this.

COVID in mice and Omicron share several mutations

“Our study calculated the average number of mutations in the five VOCs and investigated the key mutations in the viral S protein, where the infection originates. We found that the Omicron variant contains mutations at five key sites of the protein: K417, E484, Q493, Q498, and N501,” Professor Xu explains in a media release.

“This mutation profile shows that the virus has adapted to infect the cells of mice. In addition, the time-scaled phylogenetic tree shows that the Omicron and Gamma lineages were likely circulating in mid-2020, which supports the hypothesis that Omicron may have evolved in a non-human animal species. We believe that the coronavirus slowly accumulated mutations over time in mice, before it was transmitted back to humans by reverse zoonotic,” the study author continues.

mouse omicron
Five key mutations shared between omicron and mouse-adapted sars-cov-2 strains. (CREDIT: WENCHAO LIN)

If scientists confirm Omicron’s origin, it will hold major implications toward pandemic containment measures moving forward. If Omicron can jump from a rodent to a human, it’s likely that more variants are circulating among animals.

“These findings suggest that researchers should focus on SARS-CoV-2 variants isolated from wild animals, especially rodents. If Omicron is determined to have been derived from mice, the implications of it circulating among non-human hosts will pose new challenges in the prevention and control of the epidemic,” Prof. Xu concludes.

The study appears in the Journal of Biosafety and Biosecurity.