The highly contagious Omicron variant, which spread rapidly worldwide, is currently on the decline with its sub variant BA.2 in charge. Several countries, particularly in Europe and Asia, have reported a rise in BA.2.
The study from the University of Tokyo, not yet peer-reviewed, suggested that the possibility that BA.2 would be the most concerning variant to global health.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," said Kei Sato, from the varsity.
In a Twitter post, Eric Feigl-Ding, a Harvard-trained epidemiologist, wrote that the subvariant - BA.2 - is "seriously bad news".
This comes even as several countries have reopened and lifted Covid-related restrictions on view of low number of infections.
After an Omicron surge that lasted for three and a half months, the average number of global daily cases dropped for a third week in a row last week, falling back by 22 per cent to 1.97 million, News.com.au reported.
But experts are worried a new subvariant of Omicron could see a devastating surge in infections and deaths around the world just as the pandemic appears to be retreating.
"Even the World Health Organisation is getting very concerned about BA.2 variant outcompeting and displacing old Omicron," Feigl-Ding wrote on Twitter.
He wrote that news out of Denmark, where the subvariant represents 90 per cent of all new cases, suggests it is having significant health impacts.
"Here is what is happening in the country with the most BA.2 variant so far. (Denmark) has been BA.2 dominant for weeks and have now almost no mitigations either - now their excess deaths are spiking again."
Danish health officials have previously noted that BA.2 may be 1.5 times more transmissible than BA.1 based on preliminary data, but it likely does not cause more severe disease.
Maria Van Kerkhove, the World Health Organisation's Technical Lead on Covid-19, has also stated "BA.2 has a growth advantage even over BA.1".
"We need to drive transmission down. Because if we don't, we will not only see more cases, more hospitalisations, more deaths, but we will see more people suffering from Long Covid and we will see more opportunities for new variants to emerge." (agency)
Read More► Is Covid Surging Heart Problems?