The new variant, known as B.1.1.529, has been detected in small numbers in South Africa. The WHO on Friday assigned the Greek letter Omicron to the variant.
Researchers spotted B.1.1.529 in genome-sequencing data from Botswana. The variant stood out because it contains more than 30 changes to the spike protein -- the SARS-CoV-2 protein that recognises host cells and is the main target of the body's immune responses, Nature reported.
Many of the changes have been found in variants such as Delta and Alpha, and are linked to heightened infectivity and the ability to evade infection-blocking antibodies.
"There's a lot we don't understand about this variant," said Richard Lessells, an infectious-diseases physician at the University of KwaZulu-Natal in Durban, South Africa, at a press briefing organized by South Africa's health department on November 25.
"The mutation profile gives us concern, but now we need to do the work to understand the significance of this variant and what it means for the response to the pandemic," he added.
Genome sequencing and other genetic analysis showed that the B.1.1.529 variant was responsible for all 77 of the virus samples they analysed from Gauteng, collected between 12 and 20 November, the report said.
The variant harbours a spike mutation that allows it to be detected by genotyping tests that deliver results much more rapidly than genome sequencing does, Lessells said.
The researchers plan to test the virus's ability to evade infection-blocking antibodies, as well as other immune responses. The variant harbours a high number of mutations in regions of the spike protein that antibodies recognize, potentially dampening their potency.
"Many mutations we know are problematic, but many more look like they are likely contributing to further evasion," Penny Moore, a virologist at the University of the Witwatersrand in Johannesburg, South Africa, was quoted as saying.
Further, scientists suspect that Omicron could dodge immunity conferred by another component of the immune system called T cells.
But, they are more worried whether it will "reduce vaccine effectiveness, because it has so many changes?'," Aris Katzourakis, who studies virus evolution at the University of Oxford, UK was quoted as saying.
Moore said breakthrough infections have been reported in South Africa among people who have received any of the three kinds of vaccines in use there, from Johnson and Johnson, Pfizer-BioNTech and Oxford-AstraZeneca.
Researchers in South Africa also aim to study whether Omicron causes disease that is more severe or milder than that produced by other variants, Lessells said.
It is also unclear whether the variant is more transmissible than Delta, thus they said that countries where Delta is highly prevalent should be watching for signs of Omicron.