The heavily mutated Omicron variant of coronavirus is spreading faster than the Delta variant and is infecting people who have been fully vaccinated, the WHO chief has said.

"There is now consistent evidence that Omicron is spreading significantly faster than the Delta variant," WHO director-general Tedros Adhanom Ghebreyesus said at a press conference in Geneva on Monday.

"And it is more likely people vaccinated or recovered from COVID-19 could be infected or re-infected," he added.

WHO chief scientist Soumya Swaminathan said it would be "unwise" to conclude from early evidence that Omicron was a milder variant than the other strains.

The variant is successfully evading some immune responses, she said, meaning that the booster programmes being rolled out in many countries ought to be targeted towards people with weaker immune systems.

Their comments come at a time when the Omicron variant is surging in the UK, USA and other European countries such as the Netherlands. UK Prime Minister Boris Johnson has warned of a “tidal wave” of Omicron as infections spread ahead of Christmas and people are reluctant to follow Covid-appropriate behaviour.

A study by Imperial College London said last week that the risk of reinfection was more than five times higher and there is no reason to believe it is milder than Delta.

However, WHO officials said that other forms of immunity vaccinations may prevent infection and disease.

While the antibody defences from some actions have been undermined, there has been hope that T-cells, the second pillar of an immune response, can prevent severe disease by attacking the infection.

"Although we are seeing a reduction in the neutralisation antibodies, almost all preliminary analysis shows T-cell mediated immunity remains intact, that is what we really require," WHO expert Abdi Mahamud added.

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