healthHealth and Medicine

The WHO Has Identified Another COVID-19 Variant Of Interest: Mu


Tom Hale

Senior Journalist

clockSep 1 2021, 12:17 UTC

Colorized scanning electron micrograph of a cell (blue) infected with the Alpha variant strain of SARS-CoV-2 virus particles (purple). Image credit: NIAID

The World Health Organization (WHO) has said it’s keeping a close eye on a new COVID-19 “variant of interest” (VOI) known as Mu.

The Mu variant, aka B.1.621 and B.1.621.1, was first identified in Colombia in January 2021, according to the WHO. Since then, there have been a few sporadic reports of cases of the Mu variant around the globe, combined with some larger outbreaks from other countries in South America and Europe. 


As of 29 August, over 4,500 cases of the Mu variant have been documented in 39 countries. Colombia and Ecuador are the hardest hit, where 39 percent and 13 percent of cases are made up of the Mu variant, respectively. 

Mu was defined as a VOI on August 30 2021 in the latest WHO’s weekly report on the COVID-19 pandemic. This should not be confused with a “variant of concern” (VOC), such as the infamous Alpha and Delta variants, which is significantly more worrying and likely to change the course of the global pandemic. 

Under the WHO’s definition, a VOI must possess genetic changes that are predicted or known to affect virus characteristics, such as transmissibility, disease severity, immune escape, diagnostic, or therapeutic escape. On top of that, it must have caused a significant amount of transmission in multiple countries.


The Mu variant fits this definition as it has shown “potential properties of immune escape.” However, it doesn’t yet fill the criteria of a VOC. To earn this, a variant must be associated with one or more of the following: an increase in transmissibility, an increase in virulence, a change in clinical disease presentation, or a decrease in the effectiveness of public health and social measures or available diagnostics, vaccines, therapeutics.

There are currently four VOC — Alpha, Beta, Gamma, and Delta — as well as four other VOIs — Eta, Iota, Kappa, and Lambda — in circulation globally.

Meanwhile, the WHO also recently identified another “potential” variant of interest: C.1.2, a variant that’s been found in all the provinces of South Africa and a few other countries. 


For now, however, the top concern of all is the Delta variant. First identified in India back in October 2020, this variant has since become the dominant strain worldwide, responsible for an alarming rise in the COVID-19 cases, hospitalizations, and deaths around the world over the course of 2021. 

It owes this “success,” if you can call it that, to a mutation that changes a single amino acid in the viruses’ spike protein, the outer structure used to recognize and invade cells. Thanks to this small but significant mutation, the Delta is more than twice as contagious as previous variants and appears to cause a more severe illness than previous variants in unvaccinated people, according to US Centers for Disease Control and Prevention.


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