What do we know about the Mu variant?

Another variant is being mentioned in discussions about the pandemic these days. It’s called Mu (pronounced as myoo, not moo) or B.1.621, and after being identified in Colombia in January 2021, it’s now found in 39 countries across the globe.

Mu has several characteristics that make it “seem likely” (not confirmed) to infect human cells more efficiently than previous versions of the coronavirus that’s why the World Health Organization (WHO) has identified this variant as a “variant of interest” — as opposed to rather a “variant of concern” — because experts are still observing it. There have been 11 noteworthy variants to date, which the WHO has named for the letters of the Greek alphabet. Mu is the 12th.

Mu has eight mutations in its spike protein, many of which are also present in variants of concern: Alpha, Beta, Gamma, and Delta. Mu has the E484K and the K417N mutations identified in the Beta variants. The Beta variant is currently more immune-resistant than even the Delta variant but Delta is still the most transmissible and most fit version of SARS-CoV-2. Mu also has other novel mutations that have not been seen in variants before, so their consequences are not fully understood.

Though Mu has mutations suggesting that it could evade the antibodies produced by the current vaccines, that has yet to be confirmed. If more evidence backs up those early findings, it will be “promoted” to a “variant of concern.”

Cases of mu have been identified in multiple countries but it accounts for fewer than 0.1% of cases worldwide. However, in Colombia, it accounts for 39% of cases and in Ecuador, it accounts for 13% of cases. The question is: Is the Mu variant going to overtake Delta? At the moment, it seems like it is not the case but we really don’t know. 

Vaccination is still the most important step in preventing serious illness from COVID-19. Hospitals worldwide are full of mostly unvaccinated people. It takes at least two weeks for the vaccine to take effect once you receive it so get vaccinated as soon as you can.* 


Dr. Melvin Sanicas (@Vaccinologist) is a physician-scientist specializing in vaccines, infectious diseases, and global health.

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