WASHINGTON – Colorado officials on Tuesday reported the first known case in the United States of a person infected with the coronavirus variant that has been circulating rapidly across much of the United Kingdom and has led to a lockdown of much of southern England.

Scientists believe the variant is more transmissible but does not make people sicker.

The Colorado case involves a male in his 20s who is in isolation in Elbert County, about 50 miles southeast of Denver, who has no travel history, according to a tweet from the office of Gov. Jared Polis, a Democrat.

“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 statement said.

A federal scientist familiar with the investigation said the individual’s lack of known travel – in contrast with most confirmed cases outside the United Kingdom – indicates that his probably is not an isolated case. “We can expect that it will be detected elsewhere,” said the official, who spoke on the condition of anonymity to discuss the broader context of the announcement.

The Centers for Disease Control and Prevention confirmed that in a statement Tuesday afternoon, saying additional cases with the new variant will be detected in the U.S. in the coming days. The variant’s apparent increase in contagiousness “could lead to more cases and place greater demand on already strained health care resources,” the agency said.

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Researchers have now detected the more transmissible variant in at least 17 countries outside the United Kingdom, including as far away as Australia and South Korea, as of Tuesday afternoon. Officials in Canada had said they had identified two cases.

While the U.K. variant appears more contagious, it is not leading to higher rates of hospitalizations or deaths, according to a new report from Public Health England, a government agency. Nor is there any sign that people who were infected months ago with the coronavirus are more likely to be reinfected if exposed to the variant, according to the report. All available evidence indicates that vaccines, and immunity built up in the population, should be protective against this variant.

The Colorado case occurred in a county of about 27,000 people, which is currently classified, along with much of the state, in the “red” level for the coronavirus, denoting serious but not extreme risk.

Two weeks ago, several hundred people gathered at a community church in the county seat of Kiowa to consider whether to pursue legal actions against Polis and other state officials for imposing coronavirus-related restrictions, according to the Elbert County News. County commissioners and the county sheriff have declined to enforce restrictions emanating from Denver.

“I was expecting to see it in ski country first because those areas are where people from across Colorado, the U.S. and internationally gather,” said Elizabeth Carlton, an assistant professor of environmental and occupational health at the Colorado School of Public Health. The absence of apparent travel history associated with the infected person, she said, suggests he “can’t be the only case in Colorado.”

Polis, in his statement, called on Coloradans to do everything they could to prevent transmission by wearing masks, standing six feet apart when gathering with others, and interacting only with members of their immediate households.

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The arrival of the new variant “doesn’t fundamentally change the nature of the threat,” said Justin Lessler, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health. “It’s no more deadly than the virus was before, and it doesn’t look like it infects people who are immune.”

Lessler echoed others, saying he would be “astounded” if this were the only chain of transmission of the new variant in the U.S. “We know that the virus spreads easily and quickly between countries,” he said, and the fact that the infected person had no travel history indicates that “this strain has gotten here sometime in the past, and there are chains of transmission ongoing.”

The variant has a higher attack rate, according to the U.K. report, which bolsters the hypothesis that the variant has outcompeted other versions of the coronavirus and is now the dominant variant across much of the United Kingdom. Among people known to have been exposed to someone already infected with the variant, 15.1% became infected. People exposed to someone infected with the nonvariant version had a 9.8% infection rate.

That difference suggests that the variant is more transmissible, though the health agency said more investigation is needed to bolster the hypothesis.

The working theory among many scientists is that the increased transmissibility of the variant, known as B.1.1.7, is driven by mutations that have altered the spike protein on the surface of the virus. The variant has 17 mutations – eight of which alter the spike protein.

Precisely how those changes are leading to more infections is unknown. The virus may be binding more easily to receptor cells in the human body, or replicating more easily and driving higher viral loads, enhancing viral shedding by someone who is infected. Another possibility is that people are shedding virus for a longer period, raising the chances of passing along the virus.

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“Preliminary evidence suggests that the new variant does not cause more severe disease or increased mortality,”said Susan Hopkins, a senior medical adviser to Public Health England, in a statement released Tuesday.

The newly published data echo the findings in a separate study published last week, based on modeling and hospitalization data – and not yet peer-reviewed – that estimated that the variant is 56% more transmissible but does not appear to alter the lethality of the virus.

“The good news is that B.1.1.7 does not seem to cause much more severe disease, and there’s no evidence that it is managing to evade the immune system, which means vaccines are expected to protect against it,” William Hanage, an epidemiologists at the Harvard T.H. Chan School of Public Health, said Tuesday after reviewing the new report. “The bad news is that B.1.1.7 does appear to be much more transmissible.”

Officials in the United States have been signaling since last week that the new variant probably was present in this country.

“I’m not surprised,” Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Tuesday. “I think we have to keep an eye on it, and we have to take it seriously. We obviously take any kind of mutation that might have a functional significance seriously. But I don’t think we know enough about it to make any definitive statements, except to follow it carefully and study it carefully.”

Research findings on coronavirus variants have been ambiguous at times, and scientists say they are trying to extract data. Several false alarms have been sounded about virus mutations in the past. A major challenge is discerning whether a virus variant is spreading rapidly because it has a competitive advantage based on genetic and structural differences, or is simply lucky, having arrived early to a location, or leveraged a few superspreader events to gain dominance.

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But with the United Kingdom seeing a severe winter surge of infections, public officials are taking no chances and have effectively locked down southern England, including London. Other countries have barred travelers from the United Kingdom.

The United States, despite having the world’s highest number of documented infections, has a weak track record in publishing genomic sequences, the process that enables researchers to track changes in the virus. Most sequences have been published by academic or private research institutions. By contrast, the United Kingdom has a national health system with a robust surveillance system.

“The U.K. made the decision in the spring to do this,” said Neville Sanjana, a geneticist at New York University. “The U.S. has sequencing equipment and infrastructure. As with many things in this pandemic, it was not executed the way it should have been.”

All viruses mutate randomly and over time some of those mutations appear to confer some kind of advantage to the virus as in adapts to the human species. The novel coronavirus, SARS-C0V-2, mutates at a slow rate, and scientists do not think the genetic changes seen in the variant so far are sufficient to allow it to elude the vaccines now being administered to millions of people in many countries. But the coronavirus is a moving target and these mutations require surveillance.

Many scientists call the arrival of more transmissible mutations a wake-up call. “The lack of virus sequencing and case tracking in the USA is a scandal,” said Jeremy Luban, a virologist at the University of Massachusetts medical school.

Francois Balloux, who directs the Genetics Institute at University College London, predicted on Twitter that within two weeks enough data will accumulate to determine whether this new variant is more transmissible. Previously, Balloux and his colleagues combed through genome sequences, looking for evidence that common variants had increased transmissibility.

“We don’t see much,” he said, referring to a report published in Nature in November that found no signs of mutations that helped the virus to spread more easily. But he said he “wouldn’t underestimate the evolutionary potential of SARS-Cov-2.”