The Omicron variant has become the dominant type of coronavirus circulating in many countries, overtaking previous variants, such as Delta.
It can be contagious, even if you have been vaccinated. But current vaccines are still providing good protection against severe illness.
What is Omicron?
A variant is a slightly altered - or mutated - version of a virus. There are thousands of Covid variants around the world, which is to be expected because viruses mutate all the time.
Omicron is strikingly different from some other Covid variants, and the one that started the pandemic, because of the long list of genetic mutations it has undergone.
In particular, there are dozens of changes to the part of the virus targeted by the vaccine - the spike protein.
This could make it better at sidestepping some of the protection offered by vaccines, or past infections with Covid. Omicron does appear to be highly spreadable, even in those who have been fully jabbed.
There are several types or sublineages of Omicron - including BA.1.1.529 and BA.2 - that have slightly different mutations from each other.
The World Health Organization says mounting evidence suggests BA.2 is proving to be a bit more transmissible than the others.
Is it worse than other variants?
The evidence shows Omicron infections tend to be milder, since fewer people are getting sick enough with it to need hospital treatment compared to other variants.
That is largely down to the amount of protection or immunity that people have built against Covid from past infections and vaccination, rather than changes to the virus itself.
Even so, if it is more infectious it could lead to more deaths in an unvaccinated population.
Are current vaccines good enough?
Preliminary lab studies found two doses of Covid vaccine may not be enough, which is why some countries, including the UK, are giving people extra doses to boost their immunity.
UK research suggests boosters should provide good protection against severe illness.
As with other Covid variants, the risk remains highest for people who are elderly or who have significant underlying health conditions.
Although current vaccines may not be a perfect fit for Omicron, they are still the best line of defence against Covid.
They have cut the risk of severe illness against the other major Covid variants, including Delta, Alpha, Beta and Gamma.
Doctors say it is vital people get the recommended number of doses to gain maximum protection against existing and emerging variants.
How quickly could we get new vaccines against variants?
Updated versions of vaccines against Covid variants are already being designed and tested.
Manufacturers could scale up production quickly too, and regulators have already discussed how to fast-track the approval process.
What about the other variants?
The most potentially dangerous ones are called variants of concern.
UK officials have been keeping an eye on a recent descendant of the Delta variant, called AY.4.2 or "Delta plus", as well as "Domicron" or "Deltacron" Covid infections that combine Delta and Omicron.
Why do variants occur?
Viruses make carbon copies of themselves to reproduce, but they aren't perfect at it. Errors creep in that change the genetic blueprint, resulting in a new version of the virus.
If this gives the virus a survival advantage, the new version will thrive.
The more chances coronavirus has to make copies of itself in us - the host - the more opportunities there are for mutations to occur.
That's why keeping infections down is important. Vaccines help by cutting transmission as well as protecting against serious Covid illness.