W.H.O. lists Mu as ‘variant of interest.’

The World Health Organization is monitoring a new coronavirus variant called “Mu” — known by scientists as B.1.621 — and has added it to the list of “variants of interest” because of preliminary evidence it can evade antibodies.

Dr. Anthony S. Fauci, the top U.S. infectious disease expert, said the United States was also monitoring the new variant, which was first identified in Colombia in January and now makes up 39 percent of all cases there.

The W.H.O. says the variant has the potential to evade immunity provided by vaccines and antibodies, and because of that, it was listed as a “variant of interest” on Aug. 30.

Still, Mr. Fauci said it is not at all common in the United States, where the highly contagious Delta variant makes up 99 percent of all Covid-19 cases. He said the new variant “has a constellation of mutations that suggests it would evade certain antibodies,” but there is so far very little clinical data supporting that conclusion.

Mr. Fauci added that vaccines are still quite effective against variants with similar characteristics.

“Bottom line, we are paying attention to it,” he said. “We take everything like that seriously, but we don’t consider it an immediate threat right now.”