New variants of coronavirus are emerging that are more infectious than the original one that started the pandemic.
Scientists are urgently studying these mutated versions to understand what threat they pose.
Experts' concerns currently focus on a small number of new variants of coronavirus:
A UK variant that has become dominant in much of Britain and has spread to more than 50 other countries
A South Africa variant that has also been found in at least 20 other countries, including the UK
A new variant from Brazil
It's not unexpected that new variants have developed - all viruses mutate as they make new copies of themselves to spread and thrive.
There are many thousands of different versions, or variants, of Covid circulating.
Most of these differences are inconsequential. A few can even be harmful to the virus's survival. But some can make it more infectious or threatening.
It's suspected that the UK, South Africa and Brazil variants could be much more contagious or easy to catch than earlier versions.
All three have undergone changes to their spike protein - this is the part of the virus which attaches to human cells.
As a result, these variants seem to be better at infecting cells and spreading.
Experts think the UK or "Kent" strain emerged in September and may be up to 70% more transmissible or infectious, although latest research by Public Health England puts it between 30% and 50%.
The South Africa variant emerged in October, and it has more potentially important changes in the spike protein than the UK variant.
It has one of the same mutations as the UK one, plus two more that scientists think may interfere more with vaccine effectiveness. One of these may help the virus evade parts of the immune system called antibodies.
The Brazil variant emerged in July and was recently detected in four travellers arriving in Japan from Brazil. It has three key mutations in the spike protein that make it similar to the South Africa one.
The current vaccines were designed around earlier variants, but scientists are confident that they should still work against the new ones, although perhaps not quite as well.
Lab studies are underway to check this.
Vaccines train the body to attack several parts of the virus, however, not just these sections of the spike protein.
Variants could emerge in the future that are more different again.
Even in the worst case scenario, the vaccines could be redesigned and tweaked to be a better match - in a matter of weeks or months, if necessary, say experts.
As with flu vaccines, where a new shot is given each year to account for any changes in circulating flu viruses, something similar could happen for coronavirus.
There is currently no evidence to suggest that any of them cause more serious illness.
Measures such as washing your hands, keeping your distance from other people and wearing a face covering will still help prevent infections, and because the new variants spread more easily it is important to be extra vigilant.