According to CNN:
A global study has found strong evidence that a new form of the coronavirus has spread from Europe to the US. The new mutation makes the virus more likely to infect people but does not seem to make them any sicker than earlier variations of the virus.
Apparently this new mutated form of COVID-19 is the dominant form infecting people, but doesn't make them sicker than previous versions of the virus; says Erica Ollmann Saphire of the La Jolla Institute for Immunology.
The researchers have run experiments on people, animals and cells in lab dishes that show the mutated version is more common and that it's more infectious than other versions.