Some Omicron cases are reportedly experiencing “mild” symptoms and experts should have more information about the transmission of the new COVID variant within days, an epidemiologist at the World Health Organisation has said.

Speaking at a news conference, Maria van Kerkhove said one of the possible scenarios is that Omicron may become more transmissible than Delta but experts don’t yet know about its severity.

She said the WHO had seen reports of symptoms ranging from mild to severe disease.

And she said there were suggestions of increased hospitalisations from Omicron but she pointed out that this could be down to more cases.

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It comes as a key group of UK scientists warned Omicron could trigger a surge in COVID infections bigger than previous waves in the country with a risk it may overwhelm the NHS.

Experts on the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG), which advises the government, held an extraordinary meeting last week to consider the new COVID variant, known scientifically as B.1.1.529, following its detection in South Africa.

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According to a note of their meeting, which was observed by both a key Department of Health official and England’s deputy chief medical officer Jonathan Van-Tam, the group concluded the introduction of Omicron to the UK “might have very serious consequences”.

Nine more cases of Omicron have been detected in England, taking the total to 22, according to the UK Health Security Agency (UKHSA).

The people who have tested positive and their contacts are all isolating. Work is taking place to identify any links to travel to southern Africa.

Omicron cases have now been identified in the East Midlands, East of England, London, South East and North West.

A further case has been detected in Scotland, bringing the total to 10.

Meanwhile, Mike Ryan, WHO’s emergency director, said there was no evidence yet that giving booster vaccines to the whole population, including healthy people, will provide greater protection against the disease.

Boris Johnson has pledged to offer a booster COVID shot to all over-18s in the UK by the end of January.

But, responding to a question from Sky’s Tom Clarke, Dr Ryan said: “Right now there is no evidence that would suggest that boosting the entire population is going to necessarily provide any greater protection for otherwise healthy individuals against hospitalisation or death.

“The real risk of severe disease, hospitalisation and death lies in particularly at-risk and vulnerable individuals who do require protection against all variants of COVID-19.”

And his WHO colleague, Dr Soumya Swaminathan, said: “Our goal should be to protect those who have not received their primary course of vaccination.”