Stealth Omicron - also known as BA.2 subvariant - could cause more severe disease than the original version of the strain, according to a new study.
New lab experiments in Japan found BA.2 may also be more capable of escaping the immunity conferred by vaccination.
Scientists at the University of Tokyo carried out the research on animals and the study has not yet been peer-reviewed - so its quality and validity has not been independently assessed.
However, it suggests that BA.2’s “pathogenicity” is higher than that of Omicron and it is more resistant to immunity from vaccination or previous infection.
BA.2 has been rapidly spreading in Denmark and South Africa in the past few weeks - though neither have seen a substantial rise in hospitalisations.
The study found BA.2’s severity is “similar to that of an ancestral B.1.1” - otherwise known as the Alpha variant which caused a significant wave of infections in the UK in December 2020.
The authors of the study wrote: “In summary, our data suggest the possibility that BA.2 would be the most concerned variant to global health. Currently, both BA.2 and BA.1 are recognised together as Omicron and these are almost undistinguishable.
“Based on our findings, we propose that BA.2 should be recognised as a unique variant of concern, and this SARS-CoV-2 variant should be monitored in depth.”
For their study, researchers infected hamsters with the original Omicron variant and its subvariant.
They found those infected with BA.2 were worse off.
Kei Sato, a researcher at the University of Tokyo who conducted the study, told CNN the subvariant should be examined independently of Omicron.
He said countries needed to establish a specific method of detecting BA.2 due to its ability to evade detection on PCR tests.
Maria Van Kerkhove, Covid-19 technical lead at the WHO, on Thursday said the world was “seeing an increasing in proportion of sequences of BA.2”.
She added: “We are still seeing significant numbers of hospitaliations of Omicron. We are seeing significant numbers of deaths. It is not the common cold, it is not influenza. We just have to be really careful right now.”
Scientists in Denmark last month found in a study those infected with BA.2 were roughly 33 per cent more likely to infect others than BA.1.
