A new variant of coronavirus that has been circulating in South Africa is now being seen in other countries.
The UK has identified 77 cases linked to travel.
Experts are urgently studying it to understand what risk it poses.
What is the new variant?
All viruses, including the one that causes Covid-19, mutate.
These tiny genetic changes happen as the virus makes new copies of itself to spread and thrive.
Most are inconsequential, and a few can even be harmful to the virus's survival, but some can make it more infectious or threatening to the host - humans.
There are now many thousands of different versions, or variants, of the pandemic virus circulating. But experts' concerns focus on a small number of these.
One is the South African variant called 501.V2.
Is it more dangerous?
There is no evidence that new variant causes much more serious illness for the vast majority of people who become infected.
As with the original version, the risk is highest for people who are elderly or have significant underlying health conditions.
But there are concerns it can spread more readily and vaccines may not work quite as well against it.
What do experts say?
The South African variant carries a mutation called E484K, among others.
It's different to another recently discovered variant that scientists have been studying in the UK.
Both the new South African and UK "Kent" variants appear to be more contagious, which is a problem because tougher restrictions on society may be needed to control the spread.
While changes in the new UK variant are unlikely to harm the effectiveness of current vaccines, there is a chance those in the South African variant may do so to some extent, say scientists.
It is too soon to say for sure, or by how much, until more tests are completed, although it is extremely unlikely the mutations would render vaccines useless.
Scientists have tested the Pfizer Covid vaccine against one of the mutations found in the South African variant, called N501Y, using blood samples from 20 people.
In that preliminary study, vaccination appeared to work against the mutated virus.
More studies are needed though, because N501Y is not the only change the South African variant has undergone.
Dr Simon Clarke, who is an expert in cell microbiology at the University of Reading, said: "The South African variant has a number of additional mutations including changes to some of the virus' spike protein which are concerning."
The spike protein is what coronavirus uses to gain entry into human cells. It is also the bit that vaccines are designed around, which is why experts are worried about these particular mutations.
Prof Francois Balloux, from University College London, said: "The E484K mutation has been shown to reduce antibody recognition. As such, it helps the virus SARS-CoV-2 to bypass immune protection provided by prior infection or vaccination."
But even in the worst case scenario, vaccines can be redesigned and tweaked to be a better match in a matter or weeks or months, if necessary, say experts.
How far has it spread?
It is already the dominant virus variant in the Eastern and Western Cape provinces of South Africa.
At least 20 other countries including Austria, Norway and Japan, have also found cases.
What is the UK doing about it?
Travel is now banned from many countries in southern Africa, as well as Tanzania and Democratic Republic of Congo.
Public health authorities and scientists are studying the variant and will share their findings soon.