Colorado reported the first case in the U.S. of a fast-spreading variant of Covid-19 that was first detected in the U.K. and led to a widespread lockdown and travel restrictions there.

Health officials said the d the case was confirmed by a H state lab and found in a man in his 20s in Elbert County, southeast of Denver.

The man, who had no history of travel, had been placed into isolation and details of his case were reported to the U.S. Centers for Disease Control and Prevention.

“We are working to prevent spread and contain the virus at all levels,” Colorado Gov. Jared Polis said.

Health officials said they were working to identify other potential cases and contacts through contact-tracing protocols.

Authorities said the man is recovering in isolation and no close contacts had been identified so far.

The CDC said in a written statement that the agency was aware of Colorado’s report of the first U.S. case associated with the U.K. variant.

The agency said it expected there to be additional cases in the coming days.

The agency also stressed that it is normal for viruses to change through mutations and that multiple versions of the virus had been documented over the course of the coronavirus pandemic.

“Based on studies with other viruses containing similar mutations, CDC believes there will be little or no impact on immunity from natural infection or vaccination,” the agency said.

It urged anyone who was eligible to get the vaccine.

The appearance of the new variant comes at a time of surging cases around much of the U.S. On Monday, the number of new cases hit 168,000, as the death toll rose by more than 1,700 to almost 335,000, according to data compiled by Johns Hopkins University.

The number of new cases and deaths reported across the U.S. was well below the figures recorded last week, but experts say that delays in reporting because of the holiday season might be behind the drop and that numbers are likely to increase later in the week.

The seven-day average of new infections in the U.S. remains high compared with earlier points in the pandemic.

According to a Wall Street Journal analysis of Johns Hopkins data, the seven-day average of new cases hasn’t dropped below 100,000 since Nov. 4. In July, at the height of the summer surge, the national seven-day average was less than 70,000.

The Southern and Western parts of the U.S. are continuing to see increased hospitalizations, a clear indication that the surge that began in the fall hasn’t yet subsided.

In California, health officials ordered stay-at-home restrictions in two regions in the southern part of the state to be extended, as the hospital system there continues to be strained.

The Colorado state lab was the first in the country to identify the variant after a nasal-swab test came back missing certain signatures, suggesting it might be the new form of the virus.

Further genetic testing confirmed that the sample had the specific mutations found in the new variant, health officials said.

This isn’t the first time a new variant has crowded out others since the pandemic started, virologists said.

Earlier this year, another variant—one that emerged in Europe—replaced the original one from Wuhan, China, as the world’s most prevalent.

Like the new U.K. variant, that one also had a mutation in the spike protein, which helps the virus infiltrate cells.

Lab-based experiments showed that the version of the coronavirus that emerged in Europe earlier this year was better at infecting cells.

That earlier European variant only had a single change to the spike protein that could affect its function, virologists said.

The new U.K. variant has 23 mutations in total, eight of which are related to the spike protein.

Scientists have also identified a separate variant in South Africa that is becoming more common.

The new variant led the U.K. to impose new restrictions across London and the surrounding areas of England. Several countries in Europe have put travel restrictions on the U.K. in place, and the U.S. has mandated that travelers from the U.K. test negative for coronavirus within three days of arrival.

The variant’s features suggest it is more easily transmissible, National Institute of Allergy and Infectious Diseases Director Anthony Fauci said last week.

To prove that the variant is more transmissible would require additional testing in the lab, several other infectious-disease experts said.

Such tests would assess whether the new variant is stickier and attaches to and enters cells more readily than other variants.

Scientists must also study whether cells infected with the new variants produce more virus, and whether infected animals pass on the virus to uninfected ones more easily.

Even if the variant is more transmissible, there is no evidence it is any more deadly or resistant to vaccines, authorities have said.

As of Tuesday morning, about 2.1 million doses of the Pfizer Inc. - BioNTech SE and Moderna Inc. vaccines had been administered around the U.S., according to the CDC.

A spokesman for Operation Warp Speed, the federal effort to get a vaccine to market, said Tuesday that efforts remained on target and that the numbers published by the CDC reflected a lag in reporting from local health authorities.