The omicron variant of the Sars-CoV-2 virus may have evolved in mice and jumped back into humans, according to new research from researchers at the Chinese Academy of Sciences.
The origin of the variant is among the key questions for coronavirus research right now, with implications for the future of the pandemic.
Researchers have so far considered three possibilities – that it developed in a population with sparse genome testing over a period of time, or that it continued to mutate in an immunocompromised patient with a long-term infection, or that it is a product of reverse zoonosis, ie. jumped to infect an animal and jumped back into humans.
At present, several scientists have leaned towards the second scenario, as at least one variant of concern (VOC), Alpha, has most likely had such an origin.
The report by the Chinese scientists, published in the Journal of Genetics and Genomics, finds the third scenario likely due to three reasons.
First, they calculated that coronavirus naturally evolved at a rate of 0.45 mutations per month, including all the other VOCs. However, the number of mutations in the Omicron variant meant that it would have had to evolve three times as fast, with 1.5 mutations per month, which did not appear to be the natural rate of development in human hosts.
Second, and most crucially, they found that the molecular nature of mutations in the Omicron variant, when first detected in humans, was not the same as when the coronavirus evolved in humans. However, the mutations were consistent development in animals, especially mice.
Third, they found the closest relative to the Omicron variant, two B.1.1 family viruses, where they were last seen in May 2020. Until then, the mutations in these two viruses were on the expected lines.
Basically, this meant that Sars-CoV-2 took a remarkable evolutionary leap by being able to infect mice and continued to evolve within the rodent for over a year before capturing properties to infect humans. , the researchers said.
They wrote that their results suggest that Omicron’s ancestor “experienced a reverse human-to-mouse zoonotic event sometime during the pandemic (probably mid-2020) and accumulated mutations in a mouse host for more than a year before jumping back. to humans at the end of 2021 ”.
This is a scenario that has long been talked about by molecular biologists. Previously, researchers have found that mink in a farm in Denmark were infected with Sars-CoV-2 by humans, and they sent the virus back to some people.
The Chinese scientists also say that the virus could have come to be through another mechanism, by a recombination of the human variant with an animal variant, possibly in an animal host.
The results are consistent with some of the other studies that have been published recently. On January 6, this column reported on two studies, one by researchers from the University of Amsterdam, who found that Omicron had landed so far from the evolutionary tree of coronavirus that it almost created a distinct “antigenic cluster”. and another from the Imperial College of London, where researchers found that the variant was now capable of infecting domestic poultry, horseshoe bats and mice.
It also fits in with what we see globally – the Omicron variant loses a very significant feature of Sars-CoV-2: the ability to thrive in the lungs. Without it, the virus is significantly less dangerous, even though it has detected traits that make it spread faster than any other variant.
These findings reinforce the great uncertainty that now lies ahead: the variant can now evolve in any direction, and it seems to have become better at infecting even more animals, thereby expanding the reservoir of hosts, where it can continue to take evolutionary shortcuts.
The Chinese scientists allude to this. “Humans represent the largest known reservoir of Sars-CoV-2 and often come into contact with other animals, including domestic animals, pets or wild animals, that invade homes to search for food and shelter,” they said in their conclusion.
“Given the ability of Sars-CoV-2 to jump across different species, it appears that global populations will face additional animal-derived variants until the pandemic is well under control. Our study thus underscores the need for viral surveillance. and sequencing in animals, especially those in close contact with humans ”.