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. 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, according to a new report by the World Health Organization (WHO).
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.
WHO, in collaboration with national authorities, institutions and researchers, routinely assesses if variants of SARS-CoV-2 alter transmission or disease characteristics, or impact vaccine, therapeutics, diagnostics or effectiveness of public health and social measures (PHSM) applied by national authorities to control disease spread. “Signals” of potential Variants of Concern (VOCs) or Variants of Interest (VOIs) are detected and assessed based on the risk posed to global public health.
As the global public health risks posed by specific SARS-CoV-2 variants become better understood, WHO will continue to update the list of global VOIs and VOCs to support the setting of priorities for surveillance and research, and ultimately to guide response strategies.
These updates reflect virus evolution and the emergence of new variants, changing epidemiology, as well as our evolving understanding of the phenotypic impacts of variants as new evidence becomes available.
A previously designated Alert for further monitoring which subsequently meets the WHO working definition of a Variant of Interest (VOI) or Variant of Concern (VOC) can be reclassified. 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.
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.
