The World Health Organisation has designated the new B.1.1.529 Covid-19 mutation as a “variant of concern” after it moved through southern Africa and beyond and sparked travel bans that are coming into effect this week.
Scientists are still assessing the danger behind Omicron but it is thought to be highly transmissible.
The variant was labelled Omicron by the WHO, which only gives names to the most worrying strains.
What do we know so far about the Omicron Covid variant?
On Friday, there were 59 laboratory-confirmed cases of the new variant; three were in Botswana, two were in Hong Kong among people who had travelled from South Africa, and the rest were confirmed in South Africa.
By Monday, there were many more confirmed cases in the UK, Australia, Canada, the Netherlands, Germany and elsewhere.
The strain has an unusually large number of mutations and is “clearly very different” from previous variants, Tulio de Oliveira, a bioinformatics professor who runs gene-sequencing institutions at two South African universities, said on Thursday.
UK scientists first became aware of the new strain on November 23 after samples were uploaded on to a coronavirus strain tracking website from South Africa, Hong Kong and then Botswana.
“If we look at the results they had up to a week ago, less than 1 per cent of people were testing positive in lots of areas,” a senior scientist said.
“That's increased very dramatically in some parts to 6 per cent in the last few days, and so that makes me concerned quite rapidly about people who may be arriving [in the UK] now.”
How is Omicron different from the other Covid variants?
In the first three days after scientists started tracking Omicron, the mutation had been identified as having 30 different mutations.
That is twice as many as the Delta variant, which has become the most prominent variant in the UK and much of the world over the past few months.
The mutations contain features seen in all of the other variants but also traits that have not been seen before.
Richard Lessells from the University of KwaZulu-Natal in South Africa told a media briefing the strain's mutations were concerning.
“They give us concern this virus might have enhanced transmissibility, enhanced ability to spread from person to person, and might also be able to get around parts of the immune system,” he said.
The mutations could allow it to evade the protection given by prior infection or vaccination.
It is also important to determine how infectious the new strain is.
The UK Alpha variant that emerged in late-2020 was 50 per cent more transmissible and raised the country's herd immunity threshold from 66 per cent to more than 80 per cent. The Delta variant raised the stakes even higher.
Has it been classed as a Covid 'variant of concern' in the UK?
UK scientists have not yet classified Omicron as a variant of concern as they say they do not have enough evidence on the variant's levels of transmissibility — but some have said they are worried.
“B.1.1.529 has signatures of cumulative mutation indicating that it emerged in a chronic infection,” said Ravi Gupta, a professor of clinical microbiology at the University of Cambridge.
“B.1.1.529 does certainly look of significant concern based on mutations present.
“Many have been shown to impact binding by neutralising antibodies and some are known to increase the ability of virus to enter cells or to make them fuse together to allow cell-to-cell spread.”
Other Covid variants have given scientists a fright in the past but their concerns over B.1.1.529 are greater.
“Beta was all immune escape and nothing else, Delta had infectivity and modest immune escape, this potentially has both to high degrees,” said Prof Gupta.
There have been many examples of variants that have seemed worrying on paper, but which came to nothing. The Beta variant was at the top of people's concerns at the beginning of the year because it was the best at escaping the immune system, but in the end, it was the faster-spreading Delta that became dominant in the world.
Neil Ferguson, a member of the Scientific Advisory Group for Emergencies, said: “The B.1.1.529 variant has an unprecedented number of mutations in the spike protein gene, the protein which is the target of most vaccines.
“There is therefore a concern that this variant may have a greater potential to escape prior immunity than previous variants.
“However, we do not yet have reliable estimates of the extent to which B.1.1.529 might be either more transmissible or more resistant to vaccines, so it is too early to be able to provide an evidence-based assessment of the risk it poses.”
It is currently classed as a “variant under monitoring”, meaning scientists believe it may pose a future risk, but its impact is unclear.
Scientists in the UK are eager to acquire live virus cultures so it can be examined, but this takes time.
It can take seven to 10 days at least to grow enough virus that can be shared with other scientists so they can study how it mutates and changes.
Officials will now also have to wait for data to come from South Africa. The earliest they are expecting evidence to come through is two to three weeks, but it could be as long as four to six weeks.
Which countries has the UK put on its Omicron Covid red list?
First, planes from South Africa, Namibia, Lesotho, Botswana, eSwatini and Zimbabwe were stopped from flying into Britain. On Saturday, four more countries — Angola, Mozambique, Malawi and Zambia — were added to the red list.
Restrictions begin at 4am UK time (8am UAE) on November 30. Anyone allowed into the country will have to complete a PCR test and quarantine in government-approved hotels.
People arriving from red list countries who are not UK residents or Irish citizens will not be allowed into the country.
Which countries have banned flights due to the Omicron Covid variant?
Many countries have followed the UK by imposing travel restrictions on flights from southern Africa.
Japan and Israel said they would bar entry of all foreign visitors, and Morocco is suspending all incoming flights for two weeks starting November 29.
India will initially make on-arrival Covid testing mandatory for flyers from more than a dozen countries.
Australia delayed plans to relax border restrictions by at least two weeks, as the country reacts to confirmed cases of the Omicron variant.
Germany is only letting in Germans flying from South Africa, and even if they are fully vaccinated they will need to spend 14 days in hotel quarantine.
Italy, France and the Czech Republic all have some flight bans.