The dominant Omicron Covid variant is "most likely" to have come from mice, new research reveals.
Scientists now believe that it developed in mid-2020 in mice before reinfecting humans.
Back in November, the World Health Organisation (WHO) named the SARS-CoV-2 variant B.1.1.529, Omicron, as the fifth wave of concern since the start of the pandemic.
Since the surge, the Covid death toll has exceeded 150,000 in the UK, as Omicron becomes the dominant variant in many countries around the world.
As the number of hospitalisations mounts, scientists have been eager to learn more about the variant and where it comes from.
While they have been able to determine that Omicron developed from a strain that was circulating in mid-2020, they have been unable to trace any intermediate versions as Omicron evolved into its current form.
One school of thought has been that Omicron infected an animal and the mutations emerged as it spread among that species' population before it transferred back to humans, and Chinese researchers believe that this might be the case.
In research led by Jianguo Xu, a professor from the National Institute for Communicable Diseases Control and Prevention China CDC, scientists have discovered that the "most likely" intermediate host was a mouse.
According to Prof Xu, much more work is needed before their theory can be confirmed.
He said: "Our study calculated the average number of mutations in the five volatile organic compounds (VOCs) and investigated the key mutations in the viral S protein, where the infection originates."
Prof Xu, who has been a key player in understanding the Coronavirus threat, added: "We found that the Omicron variant contains mutations at five key sites of the protein: K417, E484, Q493, Q498, and N501.
“This mutation profile shows that the virus has adapted to infect the cells of mice.
"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.”
He added: “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.”
The study was published in KeAi’s Journal of Biosafety and Biosecurity.
