Sept. 2, 2021 -- The World Health Organization is tracking a new COVID-19 variant called Mu, which could be able to evade the immunity provided by vaccines and prior infections.

The variant, also known as B.1.621, was first identified in Colombia in January. It has now been detected in 43 countries and was added to the WHO’s “variant of interest” list on Monday.

“The Mu variant has a constellation of mutations that indicate potential properties of immune escape,” the WHO wrote in its weekly COVID-19 update on Tuesday.

Preliminary data suggests that the Mu variant may be able to evade antibodies at levels similar to the Beta variant, the WHO wrote, though more studies are needed. The Beta variant, also known as B.1.351, was first detected in South Africa and has shown some ability to evade vaccines.

As of Sunday, the global prevalence of the Mu variant appears to be less than .1%. But it prevalence in South America has “consistently increased,” the WHO wrote, now making up 39% of cases in Colombia and 13% of cases in Ecuador.

More than 4,700 cases of the Mu variant have been identified worldwide through genomic sequencing, according to, an open-source database operated by Scripps Research. The U.S. has identified 2,011 of these cases, with 348 in California. As of Thursday morning, only one state -- Nebraska -- had not yet reported a Mu case.

“At the moment, it looks like there’s genuine cause for concern in USA, Central America and South America, but as we saw with Delta, a potent variant can traverse the globe in the blink of an eye,” Danny Altmann, PhD, an immunologist at Imperial College London, told The Telegraph.

The WHO is monitoring nine variants with genetic mutations that could make them more transmissible, lead to more severe disease, and help them evade vaccines. The Delta variant, which is now a dominant form of the virus in the U.S. and worldwide, has led to a surge in cases and hospitalizations this summer.

In its report, the WHO said it would monitor the Mu variant for changes, “particularly with the co-circulation of the Delta variant.”

“Mu looks potentially good at immune evasion,” Altmann told The Telegraph. “For my taste, it’s a stark reminder that this isn’t by any means over. On a planet of 4.4 million-plus new infections per week, there are new variants popping up all the time, and little reason to feel complacent.”

Show Sources

WHO: “Tracking SARS-CoV-2 variants,” “Weekly epidemiological update on COVID-19 -- 31 August 2021.” “Mu Variant Report.”

The Telegraph: “WHO designates ‘Mu’ a new variant of interest.”

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