DENVER (AP) — A Colorado man who became the first reported person in the U.S. to have a COVID-19 variant that is spreading in the United Kingdom hadn’t been traveling, health officials said Monday, triggering a host of questions about how the new strain showed up in the Rockies.

Colorado officials were expected to provide more details at a news conference Wednesday about how the man in his 20s from a mostly rural area of rolling plains at the edge of the Denver metro area came down with the variant.

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

The man is in isolation southeast of Denver in Elbert County, state health officials said. Elbert County is a mainly rural area of rolling plains 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 of the variant's spread throughout the state.

Scientists in the U.K. 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.

“Being more transmissible is no good,” Bedford said, adding that the variant will make it harder to rein in the pandemic.

It’s not clear exactly why the variant appears to spread faster. It has an unusually large number of genetic changes, or mutations, particularly in areas that affect the Spike protein, according to reports by Public Health England.

And the variant has a quirk: It fails to light up one of three targets in PCR testing, which has allowed scientists in Britain to more easily sort out and compare the variant infections with those caused by the regular coronavirus. So far, they’ve found no significant difference in hospitalizations or deaths.

Public Health England deserves praise for that work, Bedford said, and the United States will struggle to accomplish anything similar because of the fragmented U.S. health care system.

“You’ll have academics and public health in different states doing their own studies,” he said. “I wouldn’t imagine being able to do anything systematically at the national scale.”

The Colorado State Laboratory confirmed the virus variant, and the Centers for Disease Control and Prevention was notified. Gov. Jared Polis announced the case Tuesday, adding urgency to efforts to vaccinate Americans.

Public health officials are investigating other potential cases and performing contact tracing to determine the spread of the variant throughout the state.

“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," Polis said.

The discovery of the new variant led the CDC to issue new rules on Christmas Day for travelers arriving to the U.S. from the U.K., 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 U.K., and southern England was placed under strict lockdown measures. Scientists say there is reason for concern but the new strains should not cause alarm.

Japan announced Monday it would bar entry of all nonresident foreign nationals as a precaution against the new strain.

New variants of the coronavirus have been seen almost since the virus was first detected in China nearly a year ago. It is common for viruses to undergo minor changes as they reproduce and move through a population. The slight modifications are how scientists track the spread of a virus from one place to another.

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.

The U.K. variant, known as B.1.1.7, has also been found in Canada, Italy, India and the United Arab Emirates.

South Africa has also discovered a highly contagious COVID-19 variant that is driving the country’s latest spike of confirmed cases, hospitalizations and deaths. The variant, known as 501.V2, is dominant among the newly confirmed infections in South Africa, according to health officials and scientists leading the country’s virus strategy.

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AP Medical Writer Carla K. Johnson in Washington state contributed.

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Nieberg is a corps member for the Associated Press/Report for America Statehouse News Initiative. Report for America is a nonprofit national service program that places journalists in local newsrooms to report on undercovered issues.