“Based on the latest round of assessments, B.1.621 was classified as a variant of interest on 30 August 2021 and given the WHO label ‘Mu’,” the organization said. “The Mu variant has a constellation of mutations that indicate potential properties of immune escape. Preliminary data presented to the Virus Evolution Working Group show a reduction in neutralization capacity of convalescent and vaccinee sera similar to that seen for the Beta variant, but this needs to be confirmed by further studies.”

The Mu variant, also known as lineage B.1.621, was first detected in Colombia in January 2021. There have been outbreaks in South America and Europe.