The man, in his 20s, is currently in isolation in Denver. Authorities say he has no travel history.
The US has reported its first case of the new COVID-19 variant that was discovered in the UK.
Governor of Colorado, Jared Polis, made the announcement on Tuesday.
The coronavirus variant was found in a man in his 20s who is in isolation southeast of Denver and has no travel history, state health officials said.
The Colorado State Laboratory confirmed the virus variant, and the Centers for Disease Control and Prevention was notified.
Scientists in the UK believe the new virus variant is more contagious than previously identified strains of the SARS-CoV-2.
The vaccines being given now are thought to be effective against the variant, Colorado health officials said in a press release.
Public health officials are investigating other potential cases and performing contact tracing to determine the spread of the variant throughout the state.
Polis and state health officials are expected to address the public on Wednesday.
The discovery of the new variant led the CDC to issue new rules on Christmas Day for travellers arriving in the US from the UK, requiring they show proof of a negative COVID-19 test.
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