Cases in Colorado and California raise questions about how mutation entered the country and whether its too late to stop it
The US has now reported multiple cases of the new and apparently more contagious variant of the coronavirus first detected in the UK, triggering concerns about how long the mutant version has been here and how widely it has spread.
The first known case was reported in Colorado on Tuesday. The person infected was later identified as a National Guardsman who had been sent to help out at a nursing home struggling with an outbreak. Health officials have said a second Guard member may also have it.