The UK, Germany and Italy all confirmed the presence of omicron on Nov 27, a day after the variant - which was first detected in South Africa - was reported to be in Belgium.
Scotland, Australia, Botswana, Denmark, Hong Kong, Israel, the Netherlands, France and Canada have all also reported cases.
Below, we have answered the key questions about the variant, the vaccines, and what the future may hold.
What are the symptoms of the omicron variant?
Fatigue and high pulse are among "unusual" symptoms spotted by Dr Angelique Coetzee, who had patients with muscle aches, "scratchy throat" and dry cough. A few had a high temperature.
She first saw the variant in a man in his early thirties with tiredness and a mild headache, but none of the usual symptoms such as a loss of taste or smell, a high temperature and continuous cough.
How is this different to previous variants?
Those who contract the dominant delta variant frequently report headaches, a sore throat and a runny nose - symptoms which are also classic features of a common cold. According to the Zoe tracker app, run by King's College London scientists, the most common symptom is a moderate to severe "pulsing" headache lasting three to five days, which painkillers don't help. Even at this early stage, omicron appears to be with milder headaches.
How will I know if I have the omicron variant?
All contacts of new variant cases will be told to self-isolate, regardless of whether or not they are fully vaccinated. In August, the Government scrapped the requirement to self-isolate after contact with a Covid case if you had taken both jabs. The UK Health Security Agency is carrying out targeted testing at locations where confirmed omicron cases were likely to have been infectious.
How many cases are there in the UK, and where are they?
As of Nov 29, 11 cases of the omicron variant have been identified in the UK. The first in Nottingham, a second in Brentwood, Essex and a third individual, who is no longer in the country but was in Westminster during their visit - and they are believed to have been contracted it in southern Africa. Following the news of Scotland reporting six cases of the new omicron variant, two further cases were confirmed in London: one in Camden and another in Wandsworth - though there is not thought to be a connection between the two.
Why is it called Omicron?
Officials at the World Health Organisation skipped two letters of the Greek alphabet when naming the latest Covid variant in order to avoid “stigmatising” China, and perhaps its premier Xi Jinping.
A WHO source confirmed the letters Nu and Xi had been deliberately avoided. Nu had been skipped to avoid confusion with the word “new” and Xi had been ducked to “avoid stigmatising the region”, they said.
Since May, new variants of Sars-COV-2 have been given sequential names from the Greek alphabet under a naming convention devised by an expert committee at the WHO. The system was chosen to prevent variants becoming known by the names of the places where they were first detected, which can be stigmatising and discriminatory.
Do vaccines protect against the variant?
Prime Minister Boris Johnson has warned that it is not yet known if vaccines will help fight against the omicron variant. He said on Saturday: "We don't yet exactly know how effective our vaccines will be against omicron, but we have good reasons for believing they will provide at least some measure of protection. And if you are boosted your response is likely to be stronger.”
This has been echoed by South African experts who have said it will take “weeks and weeks” to see if the most mutated Covid-19 variant ever detected has the ability to bypass Western vaccines.
Professor Salim Abdool Karim, former chairman of South Africa's Ministerial Advisory Committee on Covid-19, told the Telegraph it would take weeks to see if Pfizer and Johnson & Johnson vaccines will defend against the new B.1.1.529 variant.
But booster jabs are to be offered to all adults, with a recommendation by the Joint Committee on Vaccination and Immunisation expected within days, the Telegraph understands.
Mr Johnson announced that he had asked scientists to consider extending the rollout of third jabs to everyone aged 18 and over. He has also asked the JCVI to consider cutting the wait for a booster jab from six months to five and whether 12 to 15-year-olds should now be offered a second dose.
The committee is expected to recommend in favour of the mass expansion of boosters to all adults, meaning a further 13 million people would be eligible, a health source said.
Prof Anthony Harnden, deputy chair of the Joint Committee on Vaccination and Immunisation (JCVI) said accelerating the booster programme by extending the eligible age and reducing the interval between doses "will be a sensible strategy".
What is being done about the new Covid variant?
Flanked by England's Chief Medical Officer, Chris Whitty, and the Chief Scientific Officer, Sir Patrick Vallance, the Prime Minister held an unexpected press conference on Saturday and announced a series of tighter restrictions that will come into force from 4am Tuesday 30 Nov.
The new restrictions are as follows:
Anyone arriving in the UK will be asked to take a PCR test for Covid-19 by the second day and must self-isolate until they provide a negative test.
All contacts of people who do test positive with the suspected variant will have to self-isolate for 10 days. The Government confirmed that this applies to children.
The rules on face coverings are changing. They will become compulsory on public transport and in shops from next week, but not including hospitality. Teachers and pupils in Year 7 and above are now being "strongly advised" to wear masks in communal areas outside classrooms in England.
Malawi, Mozambique, Zambia and Angola will face travel restrictions from Sunday, when they will join South Africa and five other neighbouring nations on England's red list.