Cases of the Omicron Covid-19 variant are soaring in the UK at a "staggering rate" and it looks set to outpace the Delta variant as the dominant strain - it is already believed to make up most infections in London.
At least one person is known to have died with the Omicron variant in the UK and hospital admissions are rising fast.
The Omicron variant was designated as one of "concern" by the World Health Organization (WHO) in November, with one UK health official at the time calling it the “worst one we’ve seen so far”.
England's Chief Medical Officer Professor Chris Whitty said "all the things that we do know (about Omicron) are bad" despite reports of it being milder than previous variants, such as Delta or Alpha.
With cases soaring, people eager to spend Christmas with their family should "do only what you need to" to avoid catching Covid and having to self-isolate. Prof Whitty also urged people not to over-interpret reports from South Africa that Omicron causes "milder" disease than other strains, saying it would be "dangerous" to do so.
The emergence of this variant has triggered fears among some scientists that its significant number of mutations could be resistant to current vaccines.
A host of countries, including Australia, New Zealand, Sri Lanka, Israel, Brazil, Canada, Iran, Japan, Thailand and the US, joined others, including the European Union and the UK, imposing restrictions on southern African countries after scientists shared their sequencing findings, although the UK has since lifted its travel ban.
What do we know about the variant, formally named Omicron, which was first identified in southern Africa?
Which countries has the variant been spotted in?
The variant was first spotted in Botswana on November 11 and first reported to the WHO from South Africa on November 24. Belgium confirmed the first European case at the end of November, with the UK, Italy and Germany all reporting cases of the new variant within days. It has since spread to 77 countries, according to WHO.
Positive cases of the variant were recorded in South Africa following genomic sequencing and shared with scientists around the world.
Through laboratory analysis, genomic sequencing identifies a virus's genetic make-up so new variants or mutations can be detected.
How quickly is the Omicron variant spreading?
The Omicron variant of coronavirus multiplies 70 times faster than Delta in human airways, according to researchers who said their work provides the first information on how it infects people.
It is believed to be causing 200,000 new infections a day in the UK - a record 78,610 lab-confirmed Covid-19 cases were reported on Wednesday in the UK, the highest daily total since the pandemic began.
How has the World Health Organization reacted to the emergence of the new strain?
The strain was classified as a "variant of concern"- the most worrying type, like the well-known delta variant - by the World Health Organization at the end of November.
"This variant has a large number of mutations, some of which are concerning," the UN public health body said in a statement.
"Preliminary evidence suggests an increased risk of reinfection with this variant."
What precautionary action did UK take in relation to the variant?
In the wake of concerns over the threat of the Omicron variant, the government is urging children in Year 7 and above to wear face coverings in communal areas in England's schools and colleges from Monday.
Under the new guidance, all staff, visitors and pupils in Year 7 – the first year of secondary school – or above, are “strongly advised” to wear a covering, unless they are exempt.
The measures cover all education establishments including universities, as well as childcare settings such as early years care.
Face-coverings are also once again become mandatory in shops and on public transport in England.
Anyone who arrives in the country from abroad must now take a PCR test, with the expectation they will have to self-isolate until they test negative.
Double-jabbed people in England who are close contacts of Covid cases - whether Omicron or not - are now being asked to take a lateral flow test every day for seven days - or 10 days since their last contact with the person who tested positive for Covid-19 if this is earlier - to slow the spread of infections.
Originally, South Africa, Botswana, Eswatini, Zimbabwe, Namibia, Lesotho, Angola, Mozambique, Malawi and Zambia were put on the travel red list, but that has since been scrapped.
What have the prime minister and experts said?
Experts have sounded dire warnings over the new variant as people prepare to celebrate Christmas.
The strain is presenting "probably the most significant threat" of the pandemic so far, according to head of the UK Health Security Agency Dr Jenny Harries, and it could place the NHS in "serious peril". She said the Omicron variant's main threat is it "runs the risk of evading our natural and/or vaccine immunity" due to its vast number of mutations.
Prof Whitty said it is a "nailed on prospect" that hospitals will see a "substantial" increase in coronavirus patients after Christmas, adding "all the things that we do know (about Omicron) are bad".
Boris Johnson declared an “Omicron emergency” and warned people against thinking the new variant will not make them seriously ill. He said there is “no doubt” that the UK faces a “tidal wave” of Omicron infections.
Chief Scientific Advisor Sir Patrick Vallance agreed that the UK's most important defence against Omicron was currently vaccination.
Sir Patrick added that the current vaccines can in theory be easily tweaked for new variants, which could be achieved in 100 days, and that anti-viral drugs are also coming on stream.
Why does the strain having a high number of mutations concern some experts?
Viruses use what are called 'spike proteins' to enter human cells.
Some vaccines currently work by preparing our bodies to recognise these spikes and neutralise them.
The B.1.1.529 variant has 32 mutations in the spike protein, which is why some scientists are concerned the mutations may drive further waves of disease by evading the body’s defences.
Will a new vaccine need to be developed?
Pfizer and BioNTech said that in the event of a variant which could escape the effects of the vaccines, the firm expects “to be able to develop and produce a tailor-made vaccine against that variant in approximately 100 days, subject to regulatory approval”.
BioNTech, the company behind the western world's most widely used Covid vaccine, said it expects more data on the new variant within two weeks to help determine whether its shot would have to be reworked.
Professor Sir Andrew Pollard, the director of the Oxford Vaccine Group, said a new vaccine to combat Omicron could begin “very rapidly” if required.
Is it normal for viruses to mutate?
Yes. All viruses mutate - some quicker and more efficiently than others.
“SARS-CoV-2, the virus which causes Covid-19 is evolving and mutating all the time, as do all similar viruses," Prof Tom Solomon, the Director of the NIHR Health Protection Research Unit in Emerging and Zoonotic Infections, at the University of Liverpool says.
Many of these mutations will not be significant or cause for concern, says Dr Jeremy Farrar, Director of Wellcome.
“But some may give the virus an evolutionary advantage which may lead to higher transmission or mean it is more harmful."
What is a mutation?
In very simplistic terms, a virus drops off a set of instructions into a cell in the body and those instructions are followed by the cell to make more new viruses.
The instructions are then replicated and each new virus that is made receives a single copy of those replicated instructions.
But sometimes there is a mistake in these instructions.
Dr David Matthews, Reader in Virology at University of Bristol explains it is like you writing out a string of 30,000 letters by hand over and over again - the chances are you will make a typo at some point.
When one of these new viruses carrying a typo infects a new cell, the cell may look at the instructions and - unable to understand them – the process of making new viruses will fail.
But other times the instructions will still make sense and new viruses will be made that merrily replicate the mutated code and may even add more typos.