Colorado has recorded the United States' first case of the mutant Covid-19 strain that sent large parts of England into a "tier 4" lockdown earlier this month.
The Colorado man, who is in his 20s, has no travel history, suggesting to officials that he caught the strain from someone else in the US.
The mutant strain is thought to be 70 per cent more transmissible than existing strains.
While it may spread far more quickly than other strains, there's no evidence to suggest it is more lethal or that vaccines will be ineffective against it.
Colorado Governor Jared Polis revealed the news on social media.
"Public health officials are doing a thorough investigation. The individual is recovering in isolation and will remain there until cleared by public health officials," the Governor wrote.
"The individual has no close contacts identified so far, but public health officials are working to identify other potential cases and contacts through thorough contact tracing interviews."
The emergence of the mutant strain in the UK earlier this month prompted UK Prime Minister Boris Johnson to place severe restrictions on affected areas, impacting about 20 million people.
Non-essential shops, gyms, cinemas, hairdressers and bowling alleys were closed, and people were restricted to meeting one other person from another household in an outdoor public space.
"I know how disappointing this will be," Johnson said at the time. "But we have said throughout this pandemic that we must and we will be guided by the science."
Dr Anthony Fauci, the chief US health expert, warned less than a week ago that the new strain was likely circulating in the country.
"I would not be surprised, given the prevalence of this [new strain] in the UK, that we already have cases here at a low level," Fauci said.
Fauci has also said the worst period of the pandemic was likely still ahead of the US, due in no small part to the amount of travel happening over the holidays.
"We very well might see a post-seasonal – in the sense of Christmas, New Year's – surge. And as I've described it, a surge upon a surge," Fauci told CNN.
"Because if you look at the slope, the incline of cases that we've experienced as we've gone into the late fall and soon to be early winter, it is really quite troubling.
"We really are at a critical point."
The number of Covid-19-related deaths in the US stands at more than 335,000.
- additional reporting: AP