The United States on Tuesday reported its first confirmed case of COVID-19 from a variant of the coronavirus that is thought to have emerged from the United Kingdom, a fresh cause for concern as new research has found it to be more contagious than other strains.
Colorado Gov. Jared Polis announced that the state identified the case in a man in his 20s with no recent travel history. It's the first known infection from the newly identified strain in the U.S., and most experts say it's likely that more will follow.
"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," Polis said Tuesday in a news briefing.
It's not uncommon for viruses to mutate, and, indeed, several other variants of the coronavirus have already been reported. But until now, most of the mutations didn't have significant impacts on how the virus spreads or how sick infected people became.
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The new variant, dubbed VUI-202012/01, has been found to have a handful of mutations in its genetic code. Notably, it "has been predicted that these changes have the potential to make the virus more rapidly transmissible," according to the Centers for Disease Control and Prevention.
Many hospitals in the U.S. are already in crisis. The worry is that a more contagious variant of the virus could increase the number of new cases and put even more pressure on the health care system.
"If you have twice as many cases, then even if the amount of people who are sick is the same rate as it is currently, that would definitely be bad," said Dr. Diane Griffin, a professor of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health. "Anything that increases the number of cases would be bad."
The new variant has been identified in more than a dozen other countries, including France, Denmark, Japan, South Korea and Canada. But because there is no widespread effort in the U.S. to conduct regular genome sequencing of samples from across the country, it's likely that the variant is already spreading in the U.S., Griffin said.
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"I'm sure it's already spreading here and we just don't know it yet," said Griffin, who called for better monitoring of the virus's evolution as part of a committee for the National Academy of Sciences. "That's the reason we really tried to push the U.S. to have a better surveillance system, so that we are constantly monitoring what the virus is doing here and so that we would see when a new version arrived."
Scientists in the U.K. are conducting that type of forensic work now.
A new report from the country's Department of Health and Social Care found that the newly identified variant doesn't appear to be more deadly and doesn't cause more severe illness.
Although the findings provide some relief, experts say that because the new variant is more contagious, there is still an acute risk of health care systems' becoming overwhelmed in the coming weeks and months.
"One of the mutations that has been identified is known to increase the ability of the virus to bind to its receptor," Griffin said. "So it makes some sense that it would be more transmissible."
The new report, published by Public Health England, found that variant cases weren't at increased risk of reinfection and didn't have a greater case fatality rate. The study compared 3,538 COVID-19 cases, of which nearly 2,700 were identified through genome sequencing as involving the new coronavirus variant.
Severity of the disease and death aren't the only causes for concern for countries that are battling severe outbreaks of infection, like the U.K. If the virus can now spread faster through communities, for instance, more hospitalizations and deaths will naturally follow.
More research is needed, but the new variant's increased transmissibility could mean its so-called reproduction number has changed. That figure, called R-naught, is an estimate of the average number of people one COVID-19 patient is likely to infect, and it is used to represent how contagious a disease is.
Seemingly tiny changes to the reproduction number — R-naught increasing from 1.1 to 1.3, for example — can amount to big increases in potential infections because of how pathogens spread.
"An increase in something that grows exponentially (i.e. transmission) can have far more effect than the same proportional increase in something that just scales an outcome (i.e. severity)," Adam Kucharski, a mathematician and epidemiologist at the London School of Hygiene & Tropical Medicine, wrote on Twitter.
It's estimated that the new variant could be anywhere from 50 percent to 70 percent more transmissible, but more research is needed to understand why.
Another big unknown is what impact the new variant will have on immunity, if any. There is no evidence yet to suggest that the same antibodies that recognize the virus won't also work against the new variant, but it's the kind of development that keeps epidemiologists on edge.
"One of the things we're concerned about is does this change the efficacy of the vaccine?" Griffin said. "We don't have evidence that this mutation does that, but when viruses mutate, we're always worried about what it means for immunity or becoming resistant to drugs."
A separate coronavirus variant reported in South Africa is similarly thought to be more contagious, but research is still underway to better understand its characteristics.
"Lab studies take time, and we expect more info on both in the coming days and weeks," Maria Van Kerkhove, the COVID-19 technical lead for the World Health Organization, said Monday in a news briefing about the newly identified variants.
While scientists race to uncover more details about the new variants, experts say it's more important than ever to follow public health guidelines.
"All of the things we've been saying — wear a mask, practice social distancing and avoid mixing households — those mitigation strategies will continue to work," Griffin said. "The problem right now is that not everybody is following them."
This story first appeared on NBCNews.com.