A new heavily mutated version of coronavirus has been found that scientists say is of "great concern".
A new heavily mutated version of coronavirus has been found that scientists say is of "great concern".
One of most pressing questions is will vaccines still work?
There are thousands of different types, or variants, of Covid circulating across the world. That's to be expected because viruses mutate all the time.
But this new variant, called B.1.1.529 or Omicron, has experts particularly worried because it is very different to the original Covid, which current vaccines were designed to fight.
It has a long list of genetic changes - 50 in all. Of these, 32 are in the spike protein of the virus - the part which is the target of vaccines.
However, it is too soon to know how much of a threat it poses.
Current vaccines are not an ideal match so might not work quite as well, say experts.
But that doesn't mean they'll offer zero protection.
Remember, vaccines are still very effective at protecting lives by cutting the risk of severe illness against 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.
In the UK, booster jabs are being offered to:
More than 16m booster or third doses have been given so far in the UK.
Although Covid infections have been rising again across the UK, the number of hospitalisations and deaths has remained well below the levels seen in earlier waves. Experts say this is because of the success of the vaccine programme.
Scientists will be running lots of tests to check if the vaccines will hold up against this latest variant.
It is early days, but experts will study potentially important mutations that might make it more infectious and able to sidestep some of the protection given by vaccines.
And they will assess if it is causing more serious disease than other variants.
Updated versions of vaccines against Covid variants are already being designed and tested, in case they are needed at some point.
Should that time arrive, a new vaccine could be ready within weeks, to run checks on.
Manufacturers could scale up production quickly too and regulators have already discussed how to fast track the approval process.
No corners would be cut, but the whole process - from design to approval - could be much faster than when Covid vaccines were first launched.
Officials have a close watch on a few.
The most potentially dangerous ones are called variants of concern and include:
UK officials are also keeping an eye on a recent descendent of the Delta variant, called AY.4.2 or "Delta plus".
There is no evidence that any of them cause more serious illness for the vast majority of people.
As with original Covid, the risk remains highest for people who are elderly or have significant underlying health conditions.
But even so, if a variant is more infectious it will lead to more deaths in an unvaccinated population.
Vaccines offer high protection against severe illness with Covid-19, including infections caused by variants of concern. The shots also reduce the risk of infection. But they do not completely eliminate all risk.
The advice to avoid infection remains the same for all strains: wash your hands, keep your distance, wear a face covering in crowded places and be vigilant about ventilation.
Viruses make carbon copies of themselves to reproduce but they aren't perfect at it. Errors can creep in that change the genetic blueprint, resulting in a new version or variant.
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.
Experts say it is possible that the new highly altered variant B.1.1.529 may have originated in a patient whose immune system was unable to get rid of a Covid infection quickly, giving the virus more time to morph.
By Michelle Roberts
Health editor, BBC News online
