The new Covid variant found in parts of southern Africa has been designated a variant of concern by the World Health Organisation (WHO) and has been named Omicron.
WHO bosses made the announcement after the variant was detected for the first time in Europe on Friday, in Belgium.
A panel from the organisation declared it as a highly transmissible virus of concern, the same category that includes the Delta variant, the world's most prevalent.
Experts at the WHO said there is early evidence to suggest Omicron has an “increased risk of reinfection” and its rapid spread in South Africa suggests it has a “growth advantage”.
But experts, including the WHO, warned on Friday against any overreaction before the variant was better understood.
Experts have said vaccines can be tweaked to tackle new variants as they emerge.
BioNTech said in a statement that it understood “the concern of experts and have immediately initiated investigations on variant B.1.1.52”.
Moderna said it is working to advance a booster candidate tailored to the new variant and has also been testing a higher dose of its existing booster and to study other booster candidates designed to protect against multiple variants.
Novavax said it was already working on a change to its vaccine to target the new variant that could be ready for testing within weeks.
The company said data from its phase 2 trial of its Covid vaccine has “led us to believe that our vaccine is likely to provide protection against new, emerging variants.”
Health Secretary Sajid Javid said there is "huge international concern" surrounding the strain after banning flights from South Africa, Botswana, Lesotho, Eswatini, Zimbabwe and Namibia to limit its spread.
He told MPs there are concerns the variant may be more transmissible, make vaccines less effective and may affect one of the UK's Covid treatments, Ronapreve.
Ministers were facing calls to go further to prevent a wave of Omicron arriving in Britain while a Delta surge is ongoing, as Belgium became the first EU country to announce a case.
Professor John Edmunds, who advises the Government as part of the Scientific Advisory Group for Emergencies (Sage), warned that could create a "very, very, very difficult situation".
The EU, US and Canada all followed Britain's move to impose travel restrictions on visitors from southern Africa ahead of the WHO adding the strain, also known as B.1.1.529, to its highest category for concerning variants.
No cases have been detected in the UK but its arrival in Belgium - after being found in Botswana, Hong Kong and Israel - has heightened concerns.
Belgian Health Minister Frank Vandenbroucke said: “We have one case of this variant that is confirmed. It’s someone who came from abroad.”
Marc Van Ranst, who works with the Rega Institute in Belgium, tweeted that a sample was confirmed as the variant in a traveller who returned from Egypt on November 11. The patient first showed symptoms on November 22.
Earlier, Mr Javid told the Commons that experience has shown “we must move quickly and at the earliest possible moment”, adding there were concerns the variant may be more transmissible, make vaccines less effective and may affect one of the UK’s Covid treatments, Ronapreve.
He told MPs it was “highly likely” the B.1.1.529 variant had already spread from Africa to other countries.
The Government added South Africa, Botswana, Lesotho, Eswatini, Zimbabwe and Namibia to the UK’s travel red list on Thursday evening.
Passengers arriving in 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. Downing Street urged anyone who has arrived from these countries recently to get tested.
Mr Javid said discussions are ongoing over the prospect of adding further countries to the red list, saying: “We are keeping this under review and there’s very live discussions going on about whether we should and when we might add further countries, and we won’t hesitate to act if we need to do so.”
Transport Secretary Grant Shapps said the UK was “buying time” by adding countries to the list, telling BBC Radio 4’s Today programme: “I think all the history of coronavirus suggests that it is best to act quickly, determine the extent of the way that the virus interacts with vaccines, treatments, transmissibility and then give yourself a bit more time.
“It is inevitable, of course, that it will go all around the world if it is going to do so.
“So this doesn’t prevent it from coming here, but it slows things up and gives us the chance to grow the cultures and test those questions about vaccines and treatments against it.”
Dr Susan Hopkins, chief medical adviser of the UK Health Security Agency (UKHSA), said scientists are very concerned about the new variant though no cases have been detected in the UK.
It's a highly complex mutation, there's also new ones that we have never seen before
Dr Susan Hopkins
She told the Today programme that the new variant had around “30 different mutations that seem relevant – that’s double what we had in Delta (variant)”.
She added: “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.”
She said the variant was the “most worrying” seen by scientists but much was still unknown.
She added that one of the mutations in the variant is very similar to one in Alpha, which means it can be detected quite easily with PCR tests.
South African scientists fear the variant is behind a dramatic rise in cases in some regions, including Gauteng province, which includes the cities of Pretoria and Johannesburg.
“What we’re seeing in South Africa is that they were at a very, very low point with very low amount of cases being detected a day,” Dr Hopkins said.
“In a shorter period than two weeks, they have more than doubled their epidemiology picture, they are saying that the transmission rates, the R value that they have in Gauteng – around where this was first found – is now 2, which is really quite high.
“We haven’t seen levels of transmission like that since right back at the beginning of the pandemic… so that would cause a major problem if you had that high transmission with this type of virus in a population where it may evade the immune responses that are already there.”
She said South Africa was not a highly vaccinated population but it was “highly immune” due to prior infection.
Work is continuing to see whether the new variant may be causing new infection in people who have already had coronavirus or a vaccine, or whether waning immunity may be playing a role.
Asked if the variant may already be in the UK, she said: “Well, it’s always possible. We have no cases identified whatsoever yet, nothing in our genome sequencing… so overall, I think the situation is reassuring in-country, but of course, people are arriving every day.”
Other countries including Germany, Italy, France, Israel, Japan and Singapore have also restricted travel over the variant while the European Commission has recommended EU countries introduce an “emergency brake” on travel from affected countries.
However, South Africa’s foreign minister Naledi Pandor said on Friday that the UK’s decision to ban flights “seems to have been rushed”.
She said: “Whilst South Africa respects the right of all countries to take the necessary precautionary measures to protect their citizens, the UK’s decision to temporarily ban South Africans from entering the UK seems to have been rushed as even the World Health Organisation is yet to advise on the next steps.”
Professor Glenda Gray, president and chief executive of the South African Medical Research Council, told the Today programme the variant was showing between 26 and 32 changes in the spike protein of the virus and 45 amino acid changes.
The @EU_Commission will propose, in close coordination with Member States, to activate the emergency brake to stop air travel from the southern African region due to the variant of concern B.1.1.529.
— Ursula von der Leyen (@vonderleyen) November 26, 2021
She added: “We are entering into the fourth wave, and whether it’s driven by this new variant is of high concern because it may mean that people who have been infected with Delta or other variants may not be protected this time around.”
Later, England’s chief medical officer Professor Chris Whitty said there was a lot that was unknown about the new variant.
He expressed concern about further UK lockdowns if a new variant took hold, saying: “My greatest worry at the moment is that people … if we need to do something more muscular at some point, whether it’s for the current new variant or at some later stage, can we still take people with us?
“I think my overall view is I think we will. Provided you are clear with people what the logic is, provided they feel that we’re being entirely straight with them as to all the data … but I think that’s always a worry.”
The WHO panel drew from the Greek alphabet in naming the variant Omicron, as it has done with earlier, major variants of the virus.
Even though some of the genetic changes appear worrying, it was unclear if the new variant would pose a significant public health threat.
Some previous variants, such as the Beta variant, initially concerned scientists but did not spread very far.