What’s the Buzz

Mu vs. Delta Variant: What We Know

people wearing masks cartoon, with covid germs in air

Since the pandemic began in March 2020, the SARS-CoV-2 coronavirus, which causes COVID-19 disease, has spread uncontrollably worldwide. The World Health Organization (WHO), the Centers for Disease Control and Prevention (CDC), and public health officials worldwide monitor different variants of the virus based on global threats. 

Recently, the WHO named a new variant "Mu," spreading in some parts of the world. At present, the CDC does not consider the Mu variant to be a threat in the U.S., but that could change over the next few months. Please continue reading to learn more about the new coronavirus variant and how it compares to previous variants.

What are coronavirus variants?

Viruses undergo genetic mutations, i.e., they are continuously changing. A virus can undergo subtle changes in its surface proteins and antigens. Some mutations do not change the way the virus acts, while others do. For example, in the SARS-CoV-2 virus, a genetic mutation in the spike protein (which helps the virus penetrate human cells) can make the virus more contagious, cause it to spread faster, make it harder to detect, and make it more resistant to available treatments. 

SARS-CoV-2 is mutating quickly. Many new variants are being discovered, especially in countries like the United States, United Kingdom, Brazil, and India, where infection rates are high, and many people are unvaccinated. Experts are constantly trying to figure out which variants are concerning

Variants of Concern (VOCs)

The CDC lists certain COVID-19 variants as variants of concern (VOCs). Until now, all the VOCs that have been discovered have mutations in the spike protein. As mentioned, this is a critical viral structure that helps the virus infect human cells. This is concerning because scientists have used this surface protein to design the three COVID-19 vaccines authorized in the U.S. The spike protein is also the structure that the monoclonal antibody treatments target to neutralize the virus. However, thus far, none of the mutations have changed the virus enough to make the vaccines completely ineffective. 

Delta variant (B.1.617.2, AY.1, AY.2, AY.3)

First identified in India in October 2020, the Delta variant was detected in the U.S. in March 2021. It has now splintered into various sub-variants. At present, the Delta variant is the only variant of concern in the United States. It accounts for 80-95% of infections in the country. Delta is twice as contagious as the original strain from Wuhan and spreads much faster than other variants. It can cause breakthrough infection, more severe illness, and more hospitalizations. Some monoclonal antibodies used to treat COVID-19 may be less effective against Delta. However, very few people who are fully vaccinated have breakthrough infections. Nonetheless, if a fully vaccinated person does become infected with Delta, they can spread the virus to others, especially unvaccinated people.

Other variants of concern 

(These are not a threat in the U.S. at present)

Alpha variant (B.1.1.7)

First identified in the U.K. in September 2020, the alpha variant spread through Europe and appeared in the U.S. in December 2020. It was the dominant strain in the U.S. until early June 2021. 

Beta variant (B.1.351, B.1.351.2, B.1.351.3)

First identified in South Africa in October 2020, this strain appeared in the U.S. in January 2021. There are currently very few cases of this variant in the U.S.

Gamma variant (P.1, P.1.1, P.1.2)

First identified in Japan in November 2020, it was found in the U.S. in January 2021. While this variant is spreading in Brazil, it accounts for only around 1% of infections in the U.S.

Variants of Interest (VOIs)

Infectious diseases specialists at the WHO and CDC keep an eye on certain other viral strains called "variants of interest" (VOIs). Like variants of concern (VOCs), these strains can be more challenging to detect, more contagious, and more resistant to available treatments. However, they are not as widespread in multiple countries as the VOCs. 

According to the WHO and CDC, the new variants of interest are Mu, Eta, Iota, Kappa, Lambda, and an unnamed variant (B.1.617.3). None of these is currently a threat in the U.S. 

Is there a new variant called Mu?

Yes, the World Health Organization has identified a variant called Mu (B.1.621 and B.1.621.1) and lists it as a variant of interest. 

What is the MU variant of COVID-19?

The Mu variant was first detected in Colombia in January 2021. It has since been documented in more than 35 countries worldwide and 49 states in the U.S. 

In late August 2021, the CDC reported that the percentage of COVID-19 infections caused by the Mu variant was 0.2% and had fallen since July. The CDC does not currently list the Mu variant as a variant of interest. However, the WHO has designated it as a VOI due to increased infections in South America and some outbreaks in Europe.

Is the Mu strain worse than the Delta variant?

Delta continues to be the most significant threat and the most widespread coronavirus strain in the U.S. and worldwide at present. Experts say that the Mu strain is not an immediate threat. However, the Mu variant has several properties that could allow it to evade the human immune system. Meaning, it could potentially escape the protection of natural immunity after COVID-19 illness or the COVID-19 vaccines. 

The CDC reports that health officials in the U.S. are keeping a close eye on the Mu variant even though it is currently nowhere close to being the dominant strain in the country. It is too early to say whether Mu will become the next Delta strain.

How effective are COVID-19 vaccines against the Mu variant?

Early research in Italy has shown that the Mu variant is susceptible to the antibodies produced by the Pfizer-BioNTech vaccine. But the vaccine may not be as effective against this strain as it is against previous variants. More data is needed to understand this strain better and evaluate the effectiveness of the available vaccines against it.

Experts say the critical thing is for people to get vaccinated. This slows the spread of the virus down in a particular area. And while the vaccines may be slightly less effective against new variants like Delta, they are still highly effective and can prevent severe disease and death. 

Advice from the Centers for Disease Control and Prevention

The CDC says that as the COVID-19 pandemic continues, more variants are expected to emerge. The best way to reduce the spread of infection and slow the emergence of new viral mutations is through layered prevention strategies. This includes masking, social distancing, and hand washing. The CDC also recommends getting one of the COVID-19 authorized vaccines when available. Fully vaccinated people are less likely to suffer severe illness, hospitalization, and death from COVID-19.


  1. https://www.cdc.gov/coronavirus/2019-ncov/variants/variant.html
  2. https://abc7chicago.com/covid-variant-delta-mu-in-us-list-of-coronavirus-variants/10996045/