The fact that the infected man had no travel history raises the worrisome possibility that the variant is already well established in his community — and perhaps elsewhere in the United States.
The first United States case of the more contagious coronavirus variant that was initially discovered in Britain was found in Colorado on Tuesday, Gov. Jared Polis said, raising the worrisome possibility that the variant is already well established in the patient’s community — and perhaps elsewhere.
“It didn’t teleport across the Atlantic,” said William Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.
The variant was detected in a man in his 20s with no travel history, Mr. Polis said. The man was in isolation in Elbert County, southeast of Denver, he said.
Dr. Hanage said the newly reported case “should not be cause for panic.” But, he added, “it is cause to redouble our efforts at preventing the virus from getting the opportunity to spread,” he said.
Scientists are worried about variants but not surprised by them. It is normal for viruses to mutate, and most of the mutations of the coronavirus have proved minor. There’s no evidence that an infection with the variant — known as B.1.1.7 — is more likely to lead to a severe case of Covid-19, increase the risk of death or evade the new vaccines.
But the speed at which the variant seems to spread could lead to more infections — and therefore more hospitalizations.
The case identified on Tuesday came from Elbert County, Colo., which has a population of about 27,000. Colorado’s cases, deaths and hospitalizations have been steadily falling in recent weeks.
“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,” Mr. Polis said. “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.”
The Centers for Disease Control and Prevention said in a statement on Tuesday that it expected “there will be additional cases that are likely to be detected in the coming days.”
It is not clear where B.1.1.7 arose. Britain has the biggest system for sequencing the genomes of coronaviruses, which may be why the variant was first found there.
Earlier this month, British researchers observed that the variant was becoming more prevalent in parts of Britain. Their subsequent investigations suggest that the variant spreads more readily than others in circulation.
It also is not yet clear why B.1.1.7 transmits more easily. The lineage has accumulated 23 mutations since it split off from other coronaviruses. Researchers are investigating some of the mutations to see if they allow the viruses to invade cells more readily or make more copies of themselves.
Prime Minister Boris Johnson first raised the alarm over the variant spreading in England on Dec. 19. He responded to the emergence of B.1.1.7 by enforcing stronger restrictions on people’s movements and the size of gatherings.
Much of southern and eastern England, including London, has been under the highest level of restrictions. The rules require people to stay at home except for urgent travel, medical appointments and outdoor exercise. And the British Army was mobilized to help the National Health Service, the country’s health care system, set up sites to administer rapid coronavirus tests to drivers, going truck to truck to conduct tests.
“When the virus changes its method of attack, we must change our method of defense,” Mr. Johnson said at the time. “We have to act on information as we have it, because this is now spreading very fast.”
Fears of the variant’s spread from Britain last week led much of Europe to isolate the United Kingdom. As part of that effort, France imposed a 48-hour blockade of the British border that created chaos on both sides of the English Channel and warnings in Britain that some fresh goods could run short for a brief time.
As of late last week, more than 50 governments had imposed restrictions.
A new rule in the United States mandating that travelers arriving from Britain — including American citizens — show proof of a negative coronavirus test upon entry was announced late Thursday and went into effect Monday.
But experts noted that the earliest British samples of B.1.1.7 dated back to September. It was likely that the variant had already spread between countries.
It appears they were right. Cases of the variant have been identified in more than a dozen countries across the world. Health officials in Ontario, Canada, said on Sunday that they had identified two cases of the variant in a couple who had been in contact with a visitor from the U.K.
The United States sequences far fewer genomes than Britain does, which has led American scientists to suspect that the variant might already have been in the country undetected as well. On Tuesday, the United States joined the ranks of nations with B.1.1.7.
Dr. Hanage said that the United States would have to improve how it monitors the genetic sequences of circulating viruses to track their spread. It is conceivable that the new variant might have fueled recent outbreaks in the Midwest and Rhode Island, for example, but scientists do not know because public health officials have not been tracking the viruses carefully enough across the entire country.
“The United States is hobbled by the inconsistency of its approach,” he said. “Unless we turn on the lights, we won’t know it’s there.”
Because B.1.1.7 appears to be so much more contagious than other strains, British researchers have warned that current restrictions in the United Kingdom may not be sufficient. In a preliminary study, they found that schools may need to be closed and vaccination programs aggressively accelerated to prevent a surge in cases.
If B.1.1.7 takes off in the United States, it’s possible that vaccination may have to accelerate there as well. But in the first two weeks, the U.S. vaccination program is moving more slowly than expected.
“You need to be able to get whatever barriers to transmission you can out there as soon as possible,” Dr. Hanage said.
Nicholas Davies, an epidemiologist at the London School of Hygiene and Tropical Medicine, said in an interview last week that it was clear “that more rapid vaccination is going to be a really important thing for any country that has to deal with this or similar variants.”