With multiple SARS-CoV-2 genetic variants circulating in the world, the CDC has introduced a three-tiered system of classifying variants [1]. The three levels of variants are those of interest, concern, and high consequence [1]. The CDC defines the first category, variants of interests (VOI), as describing variants with greater resistance to treatment, greater resistance to vaccinations or antibodies from past infections, alternate receptor binding processes, or projected augmentations in disease severity or transmissibility [1]. At the moment, Mu, or B.1.621, is a prominent SARS-CoV-2 variant of interest [1].

Mu was first identified in Colombia in January, 2021 [2, 3]. It has since spread to at least 42 countries, with the highest prevalence rates occurring in Colombia (39%), Ecuador (13%), and some European countries [2, 4]. With only a few more sporadic cases having cropped up elsewhere in the world, the variant seems largely contained for now [4]. In the US, the Delta variant of SARS-CoV-2 remains dominant, accounting for more than 99% of cases in the country [2, 5]. Still, approximately 2,000 cases of COVID-19 attributed to the Mu variant have been identified so far [5]. Dr. Anthony S. Fauci, the US’s eminent infectious disease specialist, assured the public that Mu is “not an immediate threat right now” [5]. Regardless, he also indicated that further evidence is necessary to fully gauge the threat that Mu poses [5].

The Mu variant’s particular characteristics may affect its transmission rates, as well as its severity and ability to withstand antibodies and vaccinations [4]. Some of its spike protein mutations had not been identified before (Y145S, R346K, Y144T, and 146N insertion), while others (such as N501Y, E484K, and PG61H) are common among the more threatening variants of concern [6]. Patients infected by B.1.621 appear to experience the same symptoms as other coronavirus patients, suggesting that identification of Mu-caused disease poses no unique challenges to the medical community [5]. Furthermore, medical professionals believe that Mu does not cause more severe cases of COVID-19 than previous variants, but its multitude of mutations might mean that vaccination and antibodies will be less effective against this variant [4].

Scientists have conducted preliminary research on the ability of antibodies and vaccines to neutralize Mu, but they have yet to reach a sufficiently tested consensus. One pre-print observed Mu’s interactions with lab-derived COVID antibodies [7]. They found that the variant’s spike protein may be more resistant to neutralization than other previously identified variants of concern and interest [7]. If future experiments prove this finding to be true, Mu would have a greater chance than other variants of infecting vaccinated individuals, as well as those with antibodies from prior infection [7]. Early laboratory data suggests similar results but, again, health professionals warn that these findings are not robust enough to be readily accepted [4].

For now, medical experts in the United States believe that Mu’s likelihood of becoming a prominent and particularly dangerous variant is unlikely [4]. Regardless, Mu’s emergence emphasizes the need for unvaccinated members of the public to become vaccinated [4]. Widespread immunity is crucial to preventing variants from multiplying and to ultimately ending the COVID-19 pandemic [4].

References

[1] CDC, “SARS-CoV-2 Variant Classifications and Definitions,” Centers for Disease Control and Prevention, Updated September 17, 2021. [Online]. Available: https://www.cdc.gov/coronavirus/2019-ncov/variants/variant-info.html.

[2] S. Mishra, “The Mu variant is on the rise. Scientists weigh in on how much to worry.,” Natural Geographic, Updated September 9, 2021. [Online]. Available: https://www.nationalgeographic.com/science/article/the-mu-variant-is-on-the-rise-scientists-weigh-in-on-how-much-to-worry.

[3] B. Lovelace Jr., “WHO says it is monitoring a new Covid variant called ‘mu’,” CNBC, Updated September 1, 2021. [Online]. Available: https://www.cnbc.com/2021/09/01/who-says-it-is-monitoring-a-new-covid-variant-called-mu.html.

[4] L. Uildriks, “Mu, the new SARS-CoV-2 variant: What do we know so far?,” MedicalNewsToday, Updated September 15, 2021. [Online]. Available: https://www.medicalnewstoday.com/articles/mu-the-new-sars-cov-2-variant-what-do-we-know-so-far.

[5] A. Suliman, “Here’s what we know about the mu variant,” The Washington Post, Updated September 3, 2021. [Online]. Available: https://www.washingtonpost.com/world/2021/09/03/mu-coronavirus-variant-explained/.

[6] S. Messali et al., “A cluster of the new SARS‐CoV‐2 B.1.621 lineage in Italy and sensitivity of the viral isolate to the BNT162b2 vaccine,” Journal of Medical Virology, p. 1-3, August 2021. [Online]. doi:10.1002/jmv.27247.

[7] A. Barrett, “Mu COVID variant: All you need to know about the new UK coronavirus strain,” BBC Science Focus, Updated September 17, 2021. [Online]. Available: https://www.sciencefocus.com/news/mu-covid-variant/.