A highly infectious variant of the coronavirus was identified in the United States for the first time Tuesday, in the state of Colorado.

Scientists believe that the variant B.1.1.7, first detected in Great Britain, is more contagious than other previously identified strains of the SARS-CoV-2 variant.

"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," Colorado Governor Jared Polis said Tuesday.

Meanwhile, US President-elect Joe Biden on Tuesday criticized the pace of distribution of COVID-19 vaccines in the country.

"As I long feared and warned, the effort to distribute and administer the vaccine is not progressing as it should," Biden said in Wilmington, Delaware, adding that "it's going to take years, not months, to vaccinate the American people".

Earlier this month, Trump administration officials said they planned to have 20 million doses of the vaccine distributed by the end of the year. But according to data provided by the Centers for Disease Control and Prevention (CDC), just over 11.4 million doses have been distributed, with 2.1 million people having received their first dose.

In response, President Donald Trump tweeted: "It is up to the States to distribute the vaccines once brought to the designated areas by the Federal Government. We have not only developed the vaccines, including putting up money to move the process along quickly, but gotten them to the states."

The virus has infected more than 19.5 million people in the United States and been connected to nearly 338,000 deaths as of Tuesday, according to the Johns Hopkins University Coronavirus Research Center; both numbers are the highest in the world.

"We need to be honest — the next few weeks and months are going to be very tough, very tough for our nation. Maybe the toughest during this entire pandemic," Biden said, adding that "things are going to get worse before they get better".

The variant of the virus that causes COVID-19 also has been detected in Canada, Australia, India, Italy, South Korea, Japan and the United Arab Emirates.

In Colorado, the variant was found in a man in his 20s who is in isolation southeast of Denver in Elbert County and has no travel history, state health officials said. Elbert County is a mainly rural area at the edge of the Denver metro area that includes a portion of Interstate 70, the state's main east-west highway.

The Colorado State Laboratory confirmed the virus variant, and the CDC was notified.

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.

The vaccines being administered now — one by Pfizer and BioNTech and a second by Moderna — are believed to be effective against the variant, Colorado health officials said in a news release.

In California, the number of people hospitalized with the virus surpassed 20,000 on Tuesday, with more than 4,300 now in intensive care units. The state's data show, however, that the test positivity rate is growing at a much slower pace than it had been earlier in December.

Biden's goal of ensuring that 100 million vaccinations are administered by the end of his 100th day in office would mean "ramping up five to six times the current pace to 1 million shots a day", he said.

"We might not see improvement until we're well into March, as it will take time for our COVID response plan to produce visible progress," Biden said.

Biden also said he plans to invoke the Defense Production Act, which grants the president the power to expand industrial production of key materials or products for national security or other reasons, to accelerate production of vaccine materials.

Trump also has invoked the law during the pandemic.

Earlier in the day, Vice President-elect Kamala Harris received a COVID-19 vaccination live on television.

Biden, 78, received his first injected dose last week. Two doses are required for full protection.

The discovery of the new variant led the CDC to issue new rules on Christmas Day for travelers arriving to the US from the UK, requiring they show proof of a negative COVID-19 test.

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

Agencies contributed to this story.