Fully jabbed travellers arriving in the UK from France have to quarantine, because of the Beta variant.
The government is concerned jabs may not work as well against the variant.
French authorities say most cases are from the overseas territories of La Reunion and Mayotte, rather than mainland France.
What is the Beta variant?
Beta, also known as 501.V2 or B.1.351, has some significant genetic changes that experts are studying. It was first identified in South Africa.
All viruses, including the one that causes Covid-19, constantly mutate into new versions or variants.
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 variants can make the virus more infectious or threatening to the host - humans.
Is it more dangerous?
Some of the changes Beta has undergone involve the virus's spike protein - the part that gains the virus entry into human cells. It is also the bit that vaccines are designed around, which is why experts are concerned about these particular mutations.
Beta, along with a few other variants of coronavirus, such as Delta and Alpha, have been labelled "variants of concern". These have some worrying changes experts want to keep a very close eye on.
Beta carries a mutation, called N501Y, which appears to make it more contagious or easy to spread.
Another mutation, called E484K, could help the virus dodge a person's immune system, and may affect how well vaccines work.
There is no evidence that the South Africa variant causes more serious illness for the vast majority of people.
As with the original version, the risk is still highest for elderly people and those with significant underlying health conditions.
How good are vaccines?
Vaccines are designed around original Covid, rather than new variants.
There is some evidence they are less effective at stopping Beta infections.
But experts say they should still work to stop severe illness.
- Trials of Novavax, Janssen and the Oxford/AstraZeneca vaccines in South Africa suggest the variant can overcome some built-up immunity
- Another study, also in South Africa, found the AstraZeneca vaccine was only 10% effective against mild-to-moderate Beta infections
- A real-world data study in Israel - not yet peer-reviewed - found some people who received the Pfizer/BioNTech vaccine still caught Beta
- Lab results from Moderna suggest its shot works against the South Africa variant, although the immune response may not be as strong or prolonged
- Data on Moderna use in Qatar suggests two doses is more than 96% effective against symptomatic Covid caused by Beta
- Data from the Pfizer rollout in Qatar, suggests immunisation is 97% effective at stopping severe cases of Beta
Even in the worst case scenario, vaccines can be redesigned and tweaked to provide a better match in a matter or weeks or months, if necessary.
A trial of a new version of the AstraZeneca vaccine, designed against Beta, has begun in the UK.
Some of the other Covid vaccines are also being updated.
How far has it spread?
The Beta variant has been identified in more than 50 countries around the world.
So far, the UK has recorded just over 1,000 cases of Beta. That's a tiny fraction of all Covid cases. Another variant, called Delta, currently makes up more than 99% of Covid seen in the UK.
South Africa has reported higher cases of Beta.
What is the UK doing about it?
When it was first identified in the UK, the government recommended surge testing in the community in some regions to help stop the spread.
A system of foreign travel restrictions has also been introduced to minimise the chance of UK travellers spreading Beta as well as other variants of concern.
Although adults who were fully jabbed in the UK now no longer need to quarantine on return from amber list countries, double vaccinated travellers returning to England and Wales from France will still have to self-isolate for ten days.
They will also have to take two PCR tests to ensure they are not infectious.
UK scientists are tracking variants, including Beta, to learn more and are providing genomic sequencing help to other countries so that they can do the same.