The variant, which has not yet been named, has been spotted once in California but scientists say there are likely to be more cases. It has not been found anywhere else.
Scientists are worried because it carries mutations which appear to make it able to spread faster and also to slip past some of the immunity made from past infections or vaccines.
It was formed from the Kent variant – known scientifically as B.1.1.7 – and a Californian variant called B.1.429.
Scientists in the US claim they merged in a 'recombination event', the New Scientist reports.
This happens when two different versions of the virus infect the same cell and then swap genes while they are reproducing, giving rise to a new variant.
For a combined variant of the virus to emerge, one person must be infected with two strains of the coronavirus – likely from two separate sources – at the same time, and then the viruses must bump into each other inside the body.