After a Colorado man was determined Tuesday to have the COVID-19 variant detected in the United Kingdom, Colorado Gov. Jared Polis said the new variant “very likely exists in many states.”

“We don’t yet have a good idea how prevalent it is” in Colorado or the U.S., Polis said at a news conference Wednesday.

The man, in his 20s, is believed to be the first known case of the variant in the U.S. He has mild symptoms and is recovering, Polis said. The man has had no travel history and is in isolation, officials said.

There is another possible case of the variant that has not been confirmed, State Epidemiologist Dr. Rachel Herlihy said Wednesday.

Both individuals are National Guard personnel who were sent to a nursing home where there was an outbreak, she said. National Guard personnel arrived at the facility on Dec. 23. There's no evidence that the variant is circulating in that facility but testing is ongoing, Herlihy said.

Randy Fitzgerald, regional vice president of the nursing home, Good Samaritan Society, said in a statement, "The Colorado Department of Health and Environment have been on-site to test employees and residents for the variant. We expect to have the results in the next few days. We will continue to work closely with the state while following the CDC’s infection control measures."

The Centers for Disease Control and Prevention said it expects "there will be additional cases that are likely to be detected in the coming days."

"Viruses constantly change through mutation, and new variants of a virus are expected to occur over time," the CDC said.

The CDC added that the U.K. variant doesn't appear to cause more severe disease or mortality, but warned it does appear to have a higher transmission rate.

“I’m proud that we detected it here in Colorado as quickly as we did,” Polis said.

Some Floridians took chairs and blankets to wait in line in Fort Myers overnight for a chance to get the vaccine.

Vaccinations are being offered to high-risk front-line health care workers and people 65 and over, the Fort Myers News-Press reported.

On Tuesday, 1,019,347 million people traveled through U.S. airports, according to the TSA.

Experts predict that this Sunday will be the busiest travel day of the pandemic.

ABC News’ Sam Sweeney contributed to this report.

The United States reported 3,725 deaths on Tuesday, breaking the previous single-day total of 3,656 set on Dec. 16, according to data compiled by Johns Hopkins University.

The current numbers may not give the complete picture as the holidays have significantly impacted data, with many states having not reported numbers or have given incomplete updates.

December has already been determined to be the worst month on record for cases, hospitalizations and deaths, surpassing the high numbers seen in April.

-ABC News' Ahmad Hemingway contributed to this report.

The Centers for Disease Control and Prevention updated its guidelines for people with underlying health conditions planning to take a COVID-19 vaccine.

They CDC said that adults with underlying medical conditions -- who are more at risk of becoming seriously ill from COVID-19 -- can receive a vaccine against the virus as long as they have not had a severe allergic reaction to any of the ingredients in it.

The new guidelines state that people with HIV and those with weakened immune systems due to other illnesses or medication should be aware that information about the safety of COVID-19 vaccines for their group is not yet available. While people with HIV were included in clinical trials, more data is required to provide safety guidelines regarding the effects a vaccine could have on them. The same is true for people with autoimmune conditions.

People who have previously had Guillain-Barre syndrome or Bell’s palsy may receive a COVID-19 vaccine, though experts are still acquiring more data about their groups as well.

The CDC added that people should continue to follow coronavirus health measures -- such as wearing a mask and staying 6 feet away from others -- after receiving the shot, as experts have more to learn about the protection that COVID-19 vaccines provide under real-life conditions.