Based on the latest round of assessments, B.1.621 was classified as a VOI on 30 August 2021 and given the WHO label “Mu”. This includes the descendent Pango lineage B.1.621.1. This variant is known as 21H in Nextstrain nomenclature. 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.
Since its first identification in Colombia in January 2021, there have been a few sporadic reports of cases of the Mu variant and some larger outbreaks have been reported from other countries in South America and in Europe. As of 29 August, over 4500 sequences (3794 sequences of B.1.621 and 856 sequences of B.1.621.1) have been uploaded to GISAID from 39 countries. 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 reported prevalence should be interpreted with due consideration of sequencing capacities and timeliness of sharing of sequences, both of which vary between countries. More studies are required to understand the phenotypic and clinical characteristics of this variant. The epidemiology of the Mu variant in South America, particularly with the co-circulation of the Delta variant, will be monitored for changes.