The variant was found in a man in his 20s who is in isolation southeast of Denver in Elbert County and has no travel history, state health officials said.

Elbert County is a mainly rural area at the far edge of the Denver metro area that includes a portion of Interstate 70, the state's main east-west highway.

Colorado Politics reported there is a second suspected case of the variant in the state according to Dwayne Smith, director of public health for Elbert County. Both of the people were working in the Elbert County community of Simla. Neither of them are residents of that county - expanding the possibility that the variant has spread through the state.

The Colorado State Laboratory confirmed the virus variant, and the Centres for Disease Control and Prevention was notified.

Scientists in the UK believe the variant is more contagious than previously identified strains. The vaccines being given now are thought to be effective against the variant, Colorado health officials said in a news release.

For the moment, the variant is likely still rare in the US, but the lack of travel history in the first case means it is spreading, probably seeded by travellers from Britain in November or December, said scientist Trevor Bedford, who studies the spread of COVID-19 at Fred Hutchinson Cancer Research Centre in Seattle.

"Now I'm worried there will be another spring wave due to the variant," Bedford said. "It's a race with the vaccine, but now the virus has just gotten a little bit faster."

"There is a lot we don't know about this new COVID-19 variant, but scientists in the United Kingdom are warning the world that it is significantly more contagious. 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," Governor Jared Polis said onTuesday.

The discovery of the new variant led the CDC to issue new rules on Christmas Day for travellers arriving to the US from the UK, requiring they show proof of a negative COVID-19 test.

Worry has been growing about the variant since the weekend before Christmas, when Britain's prime minister said a new strain of the coronavirus seemed to spread more easily than earlier ones and was moving rapidly through England. The nation's first variant case was identified in southeast England.

Dozens of countries barred flights from the UK, and southern England was placed under strict lockdown measures. Scientists say there is reason for concern but the new strains should not cause alarm.

It is common for viruses to undergo minor changes as they reproduce and move through a population.

But if the virus has significant mutations, one concern is that current vaccines might no longer offer the same protections. Although that's a possibility to watch for over time with the coronavirus, experts say they don't believe it will be the case with the latest variant.

South Africa has also discovered a highly contagious COVID-19 variant that is driving the country's latest spike of confirmed cases, hospitalisations and deaths.

Australian Associated Press