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October 2, 2021

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The SARS-CoV-2 Mu Variant

COVID-19 Resources

What you need to know about the impact of the SARS-CoV-2 Mu Variant.

On behalf of the Emerging Infectious Disease Task Force 


The SARS-CoV-2 Mu variant (B.1.621) as designated by the WHO was first detected in January 2021 in Columbia, South America. Mu was elevated to a “Variant of Interest” by the WHO on August 30, 2021, but remains only as a known variant in the United States CDC system. The difference in classification here is due to the WHO global versus the CDC national missions and focus, not to any disagreement about the nature of the variant.

Current State of Variants in the United States

In the United States, the Delta (B.1.617.2) variant continues to dominate the viral population, accounting for up to 99.8% of all COVID-19 infections. The Mu variant – first detected in the United States in Los Angeles County, California – accounts for less than 0.5% of current COVID-19 infections.

Impact of the Mu Variant

The Mu variant was detected in Columbia around the start of 2021. Although it accounts for a substantial number of infections regionally in South America, it has not become the dominant variant in most locations. In addition, the Mu variant has been in the United States for several months. It is currently in all 50 states, yet it still accounts for less than half a percent of COVID-19 infections.

Current opinions are that the Mu variant has about the same or lower infectivity as the Delta variant. This should mean that although it will probably remain in the mix of the viral variants, it is not expected to replace the Delta as the dominant infecting strain based on infectivity.

Vaccine Effectiveness

The mRNA vaccines used in the United States remain effective against all known SARS-CoV-2 variants. Some variants require more antibodies to neutralize than other variants, and early evidence (not yet peer-reviewed) shows this may be true of the Mu variant. Fortunately, the level of antibody response generated by the mRNA vaccines is sufficiently robust to provide those higher levels of protection.

There is also mounting evidence that other immune elements based on T-cell programming and activation from the mRNA vaccines provide us with additional protection against SARS-CoV-2 beyond antibodies.

Building on Previous Variants and Adding Mutations

Knowledge continues to evolve regarding how mutations change the infectivity and pathogenicity of the SARS-CoV-2 virus. One generally held belief is that if a particular mutation makes one variant more dangerous, adding that mutation to another dangerous SARS-CoV-2 virus will create a “super variant” with catastrophic consequences. Although this is possible, it is not always true. As example, the Beta variants have two mutations – E484K and K417N – that make them more resistant to vaccines than the Delta variants. The Mu variants also have these mutations.

The Mu variants have not rapidly replaced the Delta variants in most populations. Initial studies suggest that it does not seem to be more resistant to the mRNA vaccines than either the Beta or Delta varieties. Therefore, having a specific mutation at a specific place on the genome does not always correlate with infectivity, pathogenicity or vaccine resistance. However, the Mu variant should be monitored based on these mutations.

The SARS-CoV-2 Mu Variant

The Mu variant has some concerning features genetically. Yet, it is not rapidly replacing the currently dominant Delta variant in most populations. It is not expected to be a dominant force in the pandemic at this time. The mRNA vaccines available in the United States continue to be effective against all known SARS-CoV-2 variants, including the Mu variant.

Promotion of primary vaccination with an mRNA vaccine, use of masks, social distancing, crowd avoidance and good handwashing continue to be the best defense during this pandemic. Find more COVID-19 resources.