As the Delta variant of COVID-19 continues its deadly sweep across the U.S., officials say that they are keeping a "very close eye" on a new variant that may be able to bypass existing coronavirus antibodies.

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said on Thursday that the U.S. is taking the variant, dubbed Mu, "very seriously," but that it hasn't taken an extensive hold in this country.

"We're keeping a very close eye on it. It is really seen here, but it is not at all even close to being dominant," Fauci said. "As you know, the Delta is more than 99% dominant."

Fauci said the Mu variant, technically known as B.1.621, has mutations suggesting "it would evade certain antibodies," potentially including those from vaccines.

"But there isn't a lot of clinical data to suggest that. It is mostly laboratory in vitro data," he added. "...We don't consider it an immediate threat right now."

The World Health Organization designated Mu a "variant of interest" this week, and said more studies need to be done to confirm whether the variant could evade existing antibodies.

Viruses are known to mutate over time as they spread, and not all variants will be able to persist. If a variant of SARS-CoV-2, the virus that causes COVID-19, proves to spread faster, cause more severe illness or lessen the effectiveness of current prevention or treatment options, it gets categorized as a more serious "variant of concern."

There are currently four "variants of concern": Alpha, Beta, Gamma and Delta. Delta, which turned up in the U.S. in the spring, is more contagious and now makes up the overwhelming majority of U.S. cases.

Mu has been detected sporadically since it was first identified in Colombia in January, WHO officials said.

Colombian health official Marcela Mercado told a local radio station on Thursday that Mu was responsible for the country's third wave of coronavirus infections from April to June. She said there were nearly 700 deaths per day during this time, and nearly two-thirds of tests from people who died came back positive for the Mu variant, according to AFP.

In the past week, Colombia has seen just under 14,000 new cases of COVID-19 and 530 new deaths, according to Johns Hopkins University. Less than 30% of the country's citizens are fully vaccinated.

There have been "some larger outbreaks" of the variant reported in South America and Europe, according to WHO. The primary regions for the variant are in Colombia and Ecuador.

"Although the global prevalence of the Mu variant among sequenced cases has declined and is currently below 0.1%, the prevalence in Colombia (39%) and Ecuador (13%) has consistently increased," the WHO said.

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