How Concerned Should We Be About the COVID-19 Mu Variant?

Authors: Authors: Alexander Stemer, M.D. Infectious Disease; Joe Kurland, M.P.H., C.I.C.; Sima Sobhiyeh, Ph.D.; Anna Foley, Leah Shaffer, Ph.D.; Sarah Ali, M.P.H.; Yasamin Saidi, Falaq Ghafur

As the COVID-19 pandemic continues and the new Mu variant appears in headlines, a common question posed to our Bloom scientific team in recent weeks has been: How concerned should we be about the COVID-19 Mu Variant?

To fully answer that question, we need to take a look at the COVID-19 genome.

COVID-19: Genomic Trends and the Mu Variant

Coronaviruses are a group of viruses named for the crown-like spikes on their surface. COVID-19 is a type of coronavirus caused by the novel SARS-CoV-2 virus. Its genome contains more RNA than any other virus group. Although the amount of RNA has not changed, when the virus that causes COVID-19 infects a person and multiplies, the large amount of reproduced RNA increases the likelihood of mutation. Between August 2020 and August 2021, approximately 99% of the original strain has been replaced, first by Beta, then Alfa, Gamma, and now Delta.

COVID Variant Trends: Delta and Mu

The current Delta variant now accounts for over 80% of cases. Specific mutations, like those in the Delta variant, allow the virus to more easily enter human cells by better binding to the angiotensin-converting enzyme 2 (ACE2) receptor and more rapidly being drawn into the cells. The stronger affinity to bind ACE2 increases entry into more cells and results in much greater virus production. It also leads to increased excretion of the virus.

The amount of the Delta variant in respiratory secretions has been cited as up to 1000 times higher than the original strain. With additional numbers of virus and faster entry into cells, the Delta variant has “increased transmissibility,” which means that it spreads more efficiently when compared with other COVID-19 variants.

The Mu variant, also known as lineage B.1.621, is a variant of COVID-19 first identified in Columbia in January 2021. It has twenty-one mutations, including nine amino acid mutations located on the virus’s spike protein. These mutations are concerning as they may increase the possibility for the virus to undergo “immune escape.” This is a term that explains what happens when the amino acid substitutions in the mutated virus disrupt the antigen structure, causing our antibodies (from either natural infection, vaccine, or monoclonal antibody administration) to lose efficacy.

COVID-19: Mu Variant Highlights & Precautions

The WHO designated the Mu variant as a variant of interest on August 30, 2021, due to the likelihood that current COVID-19 vaccines and monoclonal antibody infusions may be less effective against it. While the Mu variant has caused an increasing number of infections in Columbia — up to 39% of cases — we have not seen dramatic increases in the U.S. or the rest of the world. The Mu variant now makes up less than 0.1% of global cases. This suggests that its transmissibility has not increased and that it is less likely to cause another surge.

The concern is that as COVID-19 continues to spread globally, this process will not stop. New mutations may result in variants with increased transmissibility and enhanced immune escape. Therefore, we must continue to monitor, test, and follow the genomic trends to determine if we will need modified vaccines in the future to boost COVID-19 immunity. Meanwhile, vaccination, regular screening testing, masking, physical distancing, hand hygiene, and other basic measures that have been employed since the start of the pandemic can help contain and prevent the spread of COVID-19.

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