A new COVID-19 variant detected in New York City that's now traveled through various city neighborhoods is being watched closely by U.S. health officials, Anthony Fauci said on Monday. 

The variant, known scientifically as B.1.526, likely started off in the Washington Heights section of Manhattan, Fauci, a top adviser to President Joe Biden on the pandemic, said during a news briefing. It is one of five concerning variants now being tracked nationally by health officials.

Recent research suggests B.1.526 needs to be closely watched "for its ability to evade both monoclonal antibody and, to a certain extent, the vaccine-induced antibody," said Fauci, who also heads the National Institute of Allergy and Infectious Diseases.

"It's something we take very, very seriously," Fauci said.

The variant may have arisen in November in immunocompromised people who remained sick despite treatment over a long period of time. Fauci said infectious disease specialists are often asked whether immunocompromised people should get vaccinated.

"The answer is absolutely, yes," he said. "Absolutely, yes, because that's not only important for them for their own health. But that could be the breeding ground of the variant and the emergence of a variance."

Rochelle Walensky, the chief of the U.S. Centers for Disease Control and Prevention, again warned that COVID-19 cases appeared to have halted their decline, leveling off at what is still a high level. She said variants and the case load could combine to reignite the pandemic before vaccinations take hold.

"At this level of cases, with variants spreading, we stand to completely lose the hard-earned ground we have gained," Walensky said. "These variants are a very real threat to our people and our progress."

Last week, reports by researchers at the California Institute of Technology, Columbia University in New York and the Aaron Diamond AIDS Research Center identified the variant and said it was quickly gaining ground in the city.

Anthony West, a senior research specialist at CalTech, first raised the possibility that some portion of vaccine-induced immune response against the coronavirus "might be less effective because of the mutations" in the New York variant.

But the studies haven't yet been peer-reviewed. And Eric Topol at the Scripps Research institute in California, said that while the mutations noted in the variant probably have some impact, further significance remains unproven.

Four other mutations worrying public health officials have also emerged: From the U.K., known as B.1.1.7.; from South Africa, named B.1.351; from Brazil, named P.1; and from California, known as the B.1.427/429 variant.

The mutation that emerged from the U.K. remains on pace to become the dominant COVID-19 variant in the U.S. by the end of this month, according to the CDC. More than 2,400 cases have now been identified in 46 states. The variant that emerged from South Africa has now been seen in 53 cases in 16 states.