Three cases of the Omicron Covid-19 variant have now been detected in the UK.
The latest case was confirmed by the UK Health Security Agency (UKHSA) on Sunday, who said the person tested positive for the variant after arriving in the UK and the case is linked to travel to southern Africa.
The UKHSA said the person was in Westminster, central London, during their stay but is no longer in the UK.One of the earlier reported cases is located in Brentwood, Essex, while the other is in Nottingham, Health Secretary Sajid Javid said.
The two cases are linked and there is also a link to travel to southern Africa.
Speaking at a press conference on Saturday, Prime Minister Boris Johnson announced a raft of measures in a bid to slow the spread of Omicron in the UK.
Meanwhile, countries across the world, including the UK, have been imposing travel restrictions on the region in a bid to stop or at least slow the spread of the variant which scientists fear could be more resistant to the protection offered by vaccines.
The Omicron variant has been designated as one of "concern" by the World Health Organization (WHO), with one UK health official calling it the “worst one we’ve seen so far”.
The emergence of this variant has triggered fears among some scientists that its significant number of mutations could be resistant to current vaccines, however this has not yet been proved and experts have advised against panic.
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, in imposing restrictions on southern African countries.
So, 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.
It was first reported to the WHO from South Africa on November 24. As well as two cases in the UK, the variant has also been identified in Belgium, Hong Kong, Israel and Australia.
One case - identified in Hong Kong - involved a traveller returning from South Africa. Belgium confirmed the first European case on Friday.
On Saturday, Italy and Germany reported cases of the new variant.
An official in the German state of Hesse tweeted that the variant had likely arrived in the country after "Omicron-typical mutations" were found in someone returning from South Africa.
And Australia reported two positive cases of the Omicron variant, among a group of 14 who arrived in Sydney from southern Africa on Saturday.
Separately, 13 people who arrived in the Netherlands on two flights from South Africa on Friday have tested positive for the Omicron variant. They are among 61 people who tested positive for the coronavirus on the two flights which arrived at Amsterdam’s Schiphol Airport - all passengers are currently in isolation.
Positive cases of the variant have been recorded in South Africa following genomic sequencing.
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 in South Africa?
Experts have said there has been a rapid increase in the number of cases in South Africa over recent days.
"There has been an increase in South Africa in a number of provinces over the last few days," Deenan Pillay, professor of virology at the University College London, told ITV News.
"Only a small proportion of new Covid infections go through genetic sequencing, so the numbers could be higher."
"But it is premature to be scaremongering. We will learn quite soon whether there is any evidence of this spreading more generally," he added.
Detected cases of the Omicron variant are said to be going up particularly quickly in the South African provinces of Gauteng, North West and Limpopo.
Provincial health authorities remain on high alert, according to the National Institute for Communicable Diseases.
How has the World Health Organization reacted to the emergence of the new strain?
The strain has been classified as a "variant of concern"- the most worrying type, like the well-known delta variant - by the World Health Organization.
On Friday, the WHO said the number of cases of this variant, initially named B.1.1.529, appeared to be increasing in almost all of South Africa's provinces. "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 has the UK already taken in relation to the variant?
On Sunday, the government announced that children in Year 7 and above will be urged 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 will also again become mandatory in shops and on public transport in England from Tuesday.
Anyone who arrives in the country from abroad from 4am on Tuesday will have to take a PCR test, with the expectation they will have to self-isolate until they test negative.
In addition contacts of people who have tested positive for the Omicron variant must self-isolate for 10 days, regardless of their vaccination status, and booster vaccines will be rolled out to more people.
South Africa, Botswana, Eswatini, Zimbabwe, Namibia, Lesotho, Angola, Mozambique, Malawi and Zambia have all been put on the travel red list.
From 4am on Sunday, anyone who arrives into the UK from one of these countries must quarantine in a government-approved hotel for 10 days at a cost of £2,285 for one adult. These packages can be booked through the government's website.
Health Secretary Sajid Javid said anyone who arrives back in the UK before then should quarantine at home for 10 days and take PCR tests on days two and eight of their isolation. It is thought anyone who has been to these countries in the last 10 days will be invited to take a Covid test.
What have the prime minister and experts said?
Speaking at a Downing Street press conference on Saturday, Boris Johnson said that the current scientific understanding is that the Omicron variant “spreads very rapidly and can be spread between people who have been double-vaccinated”.
He said the extra measures were being brought in in a bid to "slow down the seeding" of the variant.
England's Chief Medical Officer Professor Chris Whitty said he expects Omicron numbers to continue to rise around the world as it spreads rapidly.
He continued that the new variant's "extensive mutations" mean “there is a reasonable chance there is some degree of vaccine escape”, but he is hopeful that current jabs can still prevent severe disease even if the vaccine does not prevent it spreading as much as would be desirable.
He added that this means there is an even greater need for everyone to have double vaccinations and boosters.
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.
The UK's Health Security Agency has said it is monitoring the situation closely.
Many scientists have said people in the UK should not panic about the new strain as it is in the early stages of its development, meaning there is little widespread evidence of its effects.
But experts have said the variant, like other viruses, can reach the UK through overseas travel.
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.
Having around twice as many mutations as the Delta variant – the Omicron variant could potentially be more transmissible and resist the protection given by prior infection or vaccination.
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. He told BBC Radio 4’s Today programme on Saturday: “The processes of how one goes about developing a new vaccine are increasingly well oiled. “So if it’s needed that is something that could be moved very rapidly.”
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.