A new variant of Covid-19, first detected in Africa, is now causing global concern.
Because of its identified mutations that risk making it both more transmissible and more likely to evade people's existing immunity, world leaders and scientists are now rushing to find out as much as they possibly can about Omicron in the shortest time possible.
The World Health Organisation says Omicron poses a very high risk of infection surges around the world while ministers have said the UK must be prepared to see more cases than the nine reported so far, as public health teams work to shut down any potential outbreaks and trace close contacts of positive tests.
But while experts question what impact the vaccines and existing antibodies can have on this most mutated strain in the coming weeks, we take a look at what we know about Omicron so far and how quickly its arrival is suddenly changing the direction of the pandemic worldwide.
What is Omicron?
Initially named B.1.1.529, Omicron was first identified in Botswana, but very quickly further cases were picked up in a number of other countries including South Africa, Hong Kong and Israel.
To begin with only a small number of cases of the strain were officially flagged by geonomic sequencing, with the majority either originating in Africa or which could be traced back to travel through the continent. But in parts of South Africa, where the strain is most prevalent, Omicron is thought to be fast becoming the dominant variant in some provinces as community transmission from person to person persists.
In the UK, there are nine cases so far.
Two in England, one in a traveller no longer in the UK and a further six in Scotland. Among the cases in Scotland, not all positive tests were as a result of recent travel or contact with a person who has been away, so there could be some small amounts of community transmission already happening there, First Leader Nicola Sturgeon has said.
What has scientists concerned?
Investigations have found that the B.1.1.529 strain has 32 mutations to its spike protein - the part of the virus the majority of vaccines use to prime the immune system against Covid 19 - leaving experts to question the risk it might pose to vaccines if it were to continue to spread in large numbers.
Dr Tom Peacock, a virologist at the Department of Infectious Disease based at Imperial College London, posted details about the new variant on Twitter last week warning that its 'horrific' spike profile means it should be closely watched.
In a series of tweets he wrote: "Worth emphasising this is at super low numbers right now in a region of Africa that is fairly well sampled, however it very very much should be monitored due to that horrific spike profile (would take a guess that this would be worse antigenically than nearly anything else about."
Is Omicron more deadly?
The impact Omicron will have on coronavirus numbers worldwide is yet to be seen because it is only just emerging. And as we know with Covid19, there is always a time lag between case numbers, the numbers of patients who go on days or weeks later to require hospital treatment for serious illness and those who don't survive.
As a result of geonomic sequencing, which looks at the genetic make up of positive covid tests to identify new variants or mutations in circulation, scientists often identify strains of the virus that never grow to be any more than a small number of cases or they find a strain, that despite changes its appearance, behaves no differently to others that have been identified before or which are currently in circulation.
Reports emerging from South Africa quote some medical staff as saying Omicron doesn't appear to be leading to more severe infection and that so far people are experiencing mild symptoms but scientists and world leaders need more evidence to support this.
It is for this reason that close contacts of positive Omicron cases in the UK will now be required to self isolate for 10 days - a rule which hasn't been imposed here since the summer for people who are either under the age of 18 or double vaccinated - and greater contact tracing to identify contacts is now taking place.
And even if the virus is no more deadly - if it is more contagious than our current main strain Delta - it will still have the power to cause significant disruption to every day life.
What are the new restrictions?
As health experts and governments attempt to buy themselves more time to find out as much as they can about Omicron, a number of restrictions are being reintroduced to slow the spread of the virus.
From Tuesday (November 30) face masks will need to be worn once again by law in shops and on public transport in England - something already in place in Wales, Scotland and Northern Ireland.
Secondary schools, colleges and universities have also been told to reintroduce mask wearing in communal spaces for students in Year 7 and above.
In terms of travel, everyone arriving into the UK is now required to take a PCR test after their second full day of being in the country, while international arrivals from the UK's new red list will once again be required to complete a 10-day quarantine stint in government-managed hotels.
The UK is not the only country to re-impose travel restrictions on international arrivals - with most parts of Europe alongside countries such as the US, New Zealand, Japan, Thailand and Morocco all bringing in new tougher rules in light of Omicron's arrival.
The latest round of restrictions will be reviewed again in three weeks.
What about vaccines?
Despite widespread speculation Omicron could be more resistant to vaccines the government message remains the same - and that anyone entitled to either their first or second jab, or third booster, takes up the offer as soon as possible in order to obtain the highest protection possible against any coronavirus strain.
The government also announced on Monday afternoon plans to expand the booster programme - with the offer of a third jab to all adults alongside shortening the gap between second and third doses in order to roll out injections more quickly and give additional protection to those who might be seeing immunity from their first two doses wane slightly some months on.
While it is hard to know exactly how Omicron might behave in a population as highly vaccinated as the UK, ministers have told families that as it stands, they should continue to plan for a normal Christmas.