WHO's Technical Advisory Group on SARS-CoV-2 Virus Evolution (TAG-VE), an independent group of experts, met Friday to discuss the variant, a WHO statement said.
The advisers recommended WHO designate the variant as "of concern," referencing the variant's large number of mutations, the possibility of increased risk of reinfection and other evidence.
A number of studies are underway, and WHO will update member states and the public as needed, the WHO statement said.
WHO called on countries to enhance their surveillance and sequencing efforts to better understand coronavirus variants.
"Initially it looked like some cluster outbreaks, but from yesterday, the indication came from our scientists from the Network of Genomic Surveillance that they were observing a new variant," Joe Phaahla, South Africa's Minister of Health, said Thursday, stressing it is currently unclear where the variant first emerged.
South African officials initially said there was one confirmed case in a traveler from South Africa to Hong Kong. Then Hong Kong health authorities on Friday identified a second case of the B.1.1.529 variant among returning travelers on the same floor of a designated quarantine hotel.
Also on Friday, the Belgian government said one individual who had recently arrived from abroad, and was not vaccinated, had tested positive for the new variant, marking the first case in Europe.
Tulio de Oliveira, the director of South Africa's Center for Epidemic Response and Innovation, said the variant has "many more mutations than we have expected," adding it is "spreading very fast, and we expect to see pressure in the health system in the next few days and weeks."
Viruses, including the one that causes Covid-19, mutate regularly and most new mutations do not have significant impact on the virus's behavior and the illness they cause.
Dr. Ashish Jha, dean of Brown University's School of Public Health, told CNN the variant was "acting differently," however, and it "looks like it's much more contagious than even the Delta variant."
Acting on advice from US health officials, US President Joe Biden will restrict travel from South Africa, Botswana, Zimbabwe, Namibia, Lesotho, Eswatini, Mozambique and Malawi, administration officials told CNN.
This does not apply to American citizens and lawful permanent residents. As with all international travelers, they must still test negative prior to travel.
What we know about the new variant
Lawrence Young, a virologist and a professor of molecular oncology at Warwick Medical School in the United Kingdom, said the Omicron variant was "very worrying."
"It is the most heavily mutated version of the virus we have seen to date. This variant carries some changes we've seen previously in other variants but never all together in one virus. It also has novel mutations," Young said in a statement.
has a high number of mutations, about 50 overall. Crucially, South African genomic scientists said Thursday more than 30 of the mutations were found in the spike protein
-- the structure the virus uses to get into the cells they attack.
Neil Ferguson, the director of the MRC Centre for Global Infectious Disease Analysis at Imperial College London, said in a statement the number of mutations on the spike protein was "unprecedented."
"The spike protein gene [is] the protein which is the target of most vaccines. There is therefore a concern that this variant may have a greater potential to escape prior immunity than previous variants," Ferguson said.
Sharon Peacock, a professor of Public Health and Microbiology at the University of Cambridge, said while the overall number of Covid-19 cases is relatively low in South Africa, there has been a rapid increase in the past seven days.
She said while 273 new infections were recorded on November 16, the figure had risen to more than 1,200 cases by November 25, with more than 80% coming from Gauteng province.
"The epidemiological picture suggests that this variant may be more transmissible, and several mutations are consistent with enhanced transmissibility," Peacock said in a comment shared by the UK's Science Media Centre.
She added while the significance of the mutations and their combination is unknown, some of those present in the latest variant have been associated in others with immune evasion.
Jha, too, said scientists were concerned by the speed with which the Omicron variant has taken off. "This one has become dominant very quickly in South Africa, in the regions where it's been found, within a matter of days to weeks as opposed to months," he told CNN.