The Omicron coronavirus variant, reported in more than 60 countries, poses a "very high" global risk, with some evidence that it evades vaccine protection but clinical data on its severity is limited, the World Health Organization says.
Considerable uncertainties surround Omicron, first detected last month in southern Africa and Hong Kong, whose mutations may lead to higher transmissibility and more cases of COVID-19 disease, the WHO said in a technical brief issued on Sunday.
"The overall risk related to the new variant of concern Omicron remains very high for a number of reasons," it said, reiterating its first assessment of Nov. 29.
It added there were early signs that vaccinated and previously infected people would not build enough antibodies to ward off an infection from Omicron, resulting in high transmission rates and "severe consequences".
It remains unclear for now whether the new lineage is also inherently more contagious than the dominant Delta variant, which would fuel its spread further, WHO warned.
Corroborating the WHO's assessment, University of Oxford researchers published a lab analysis on Monday saying that two two-dose COVID-19 vaccine regimens do not induce enough neutralising antibodies against Omicron.
While the antibody defences from courses from AstraZeneca vaccine and BioNTech/Pfizer have been undermined, there is hope that T-cells, the second pillar of an immune response, can at least prevent severe disease by attacking infected human cells.
The Oxford researchers said there was currently no evidence of Omicron causing more severe disease.
Pfizer and BioNTech have said two shots of their vaccine may still protect against severe disease, because its mutations were unlikely to evade the T-cells response.
The WHO cited some preliminary evidence that the number of people getting reinfected with the virus has increased in South Africa.
While early findings from South Africa suggest that Omicron may be less severe than the Delta variant - currently dominant worldwide - and all cases reported in the Europe region have been mild or asymptomatic, it remained unclear to what extent Omicron may be inherently less dangerous, it said.
"More data are needed to understand the severity profile," it said. "Even if the severity is potentially lower than for the Delta variant, it is expected that hospitalisations will increase as a result of increasing transmission. More hospitalizations can put a burden on health systems and lead to more deaths."
Further information was expected in coming weeks, it added, noting the time lag between infections and outcomes.
