Covid variant: What happens when coronavirus mutates?

Surge testing is being used in parts of the UK to help control the spread of new Brazil and South Africa variants.

The Kent variant is now the most commonly found strain of Covid-19 in the UK and it's more easily spread than the first variant, which arrived in Spring 2020.

Scientists are looking at tweaking the coronavirus vaccines to make sure they continue to be highly effective.

But where are these variants coming from? And why do viruses mutate?