Colorado Governor Jared Polis said the state has found the first known US case of a highly contagious Covid-19 variant first detected in the UK.
Mr Polis said the variant, B117, was found in a young man with no recent travel history, which suggests it is already in the US spreading undetected.
He said the Centres for Disease Prevention and Control has been notified and the man is in isolation.
Mr Polis said the case was being investigated further, and contact tracing carried out.
It could mean there is a community spread of the variant.
“That Colorado has detected this variant first in the nation is a testament to the sophistication of Colorado’s response and the talent of [the state’s] scientist and lab operations,” said Jill Hunsaker Ryan, executive director of the Colorado Department of Public Health and Environment.
The US is among several countries investigating the mutation. Initial research suggests it is more contagious than other variants.
Dozens of countries banned air travel to and from Britain as a result.
Some countries also imposed additional quarantine requirements on those who arrived from the UK.
The Trump administration initially did not join the global trend, leaving states to decide how to respond.
British Airways, Virgin Atlantic and Delta say travellers from the UK must provide negative Covid-19 test results before boarding US-bound flights, after a request from New York Governor Andrew Cuomo.
On December 24, the CDC issued new rules requiring negative coronavirus tests for travellers arriving from Britain to any American city.
“The health and safety of Coloradans is our top priority and we will closely monitor this case, as well as all Covid-19 indicators, very closely,” Mr Polis said.
“We are working to prevent spread and contain the virus at all levels.”
