The World Health Organization (WHO) has named the new coronavirus variant detected in South Africa, and designated it a variant of concern (VOC).
B.1.1.529 will be called the Omicron variant, WHO experts announced, following a meeting convened to assess the situation amid mounting concern.
Cases have so far been confirmed in South Africa, Botswana, Hong Kong, Belgium and Israel.
However it is highly likely that it has spread further without so far being detected.
The WHO said in a statement: "This variant has a large number of mutations, some of which are concerning.
"Preliminary evidence suggests an increased risk of reinfection with this variant, as compared to other VOCs.
"The number of cases of this variant appears to be increasing in almost all provinces in South Africa."
Scientists have expressed concern about the “horrific” number of mutations on the virus' spike protein in the variant, which they fear could make it highly transmissible, more deadly, and make vaccines less effective.
In response to the outbreak, the UK placed South Africa, Botswana, Lesotho, Eswatini, Zimbabwe and Namibia on its travel red list.
Passengers arriving into the UK from these countries from 4am on Sunday will be required to book and pay for a government-approved hotel quarantine for 10 days.
Making an emergency statement in the House of Commons on Friday, Sajid Javid said there were no confirmed UK cases of the new coronavirus variant, but that the government is concerned it could “pose a substantial risk to public health”.
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Transport secretary Grant Shapps said the UK was “buying time” by limiting travel from countries where cases of the variant have been confirmed.
Other countries including Germany, Italy, Israel and Singapore have also restricted travel over the variant.
Earlier on Friday, WHO's Special Envoy on COVID-19, David Nabarro, said it was appropriate to be concerned about the new coronavirus variant.
"My own view is that really it is appropriate to be concerned about this," he told the BBC.
"I'll tell you why. The virus looks like it will have greater capacity to evade the defences that we've all built up as a result of the vaccinations we've received since the beginning of this year."
Dr Susan Hopkins, chief medical adviser of the UKHSA, told BBC Radio 4’s Today programme the variant was the "most worrying" so far detected – but stressed that much is still unknown about its properties.
She said: "If we look at those mutations, there’s mutations that increase infectivity, mutations that evade the immune response both from vaccines and from natural immunity, mutations that cause increased transmissibility.
“It’s a highly complex mutation, there’s also new ones that we have never seen before.”
A scientist advising the government has said the Omicron variant is a “huge worry” and could evade current vaccines.
Professor John Edmunds, a member of the Scientific Advisory Group for Emergencies (Sage), told BBC Radio 4’s PM programme: “This is a huge worry, unfortunately.
“The molecular data is extremely worrying. The molecular data would point to that perhaps this thing might be able to evade the immune response.”
Asked if Omicron could be resistant to vaccines, he said: “Yes, that’s the great fear and all the data suggests that it might well be able to evade the immune response, what we don’t know is to what extent. Our fears are it would do so to a large extent.”
He warned that a “rapid rise” in the variant in South Africa has come after a “huge wave” of the Delta variant.
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