Written by: Samuel Go

A COVID-19 variant from Colombia called Mu has spread across the entire US, leading the World Health Organization to declare it a variant of interest. A team of Japanese researchers published an unreviewed preprint paper on September 7 detailing how the Mu variant was more resistant to vaccines than all other COVID-19 strains, including Delta. However, Delta remains much more competitive and dangerous than any other COVID-19 variant, including Mu.

How is this variant different from Delta?

Mu was first sequenced in Colombia in January 2021 and declared a variant of interest on August 30, 2021, by the World Health Organization. It is still a new variant, so there is not much known about the differences between it and widespread COVID-19 strains like Delta. A group of Japanese researchers did discover that Mu was more resistant to vaccines than any other strain of COVID-19, including Delta. However, the research paper has still not been peer-reviewed as of September 11, 2021, so the findings are not conclusive. There is still not much information about Mu as there have been so few studies on the variant.

Figure 1

(A) Daily new Colombian COVID-19 cases on the y-axis. Date on the x-axis. Percentage of cases caused by a particular strain as colored bars.

(B) Virus neutralization assay comparing the geometric mean titers of previously infected people. A lower number on top of the bars means that a virus can only be detected under a more concentrated blood serum sample and vice versa. A lower number on top also means that the virus can better evade antibodies targeting it compared to other viruses.

(C) Same as (B) but with vaccinated people

Source: BioRxiv 

How concerned should we be about Mu?

From the information available, Mu may be more genetically resistant to COVID-19 immunity from vaccines and infection than other strains. As a result, it may cause breakthrough infections for those who have already been infected by other COVID-19 strains. However, vaccines still protect against serious cases of COVID-19 from even the most genetically resistant strains like Beta and potentially Mu.

Over the summer, the percentage of total active Colombian COVID-19 cases caused by Mu fell by 6 percent while the percentage of Delta cases rose by 13 percent. The data indicates that Mu is nowhere near Delta in terms of infectiousness. Even though Mu is more resistant to treatment, it is nowhere near as concerning as Delta, which is much more transmissible than Mu and therefore able to outcompete it anywhere.

Conclusion

Currently, nobody knows whether Mu will become the new “super-variant.” Mu may be worse than Delta because it is more resistant to vaccination and treatment, but Delta has already outcompeted Mu everywhere, even in Colombia where Mu originated. The best course of action to stop COVID-19, according to many health experts, is to focus completely on containing the spread of Delta through public health efforts like vaccination campaigns, enforcing social distancing, and mask mandates.

References and Sources

Cheng, M. (2021, September 8). What is the mu variant of the coronavirus?. Retrieved September 11, from https://apnews.com/article/lifestyle-health-europe-united-states-coronavirus-pandemic-9bc7b53dc5c119ddcfe7e124ae329ed4 

Lovelace Jr., B. (2021, September 7). WHO says delta remains the ‘most concerning’ Covid variant despite emergence of mu. Retrieved September 11, 2021, from https://www.cnbc.com/2021/09/07/mu-covid-variant-who-says-delta-remains-most-concerning.html 

McNamara, D. (2021, September 9). Q&A: How Concerned Should We Be About the Mu Variant?. Retrieved September 15, from https://www.webmd.com/lung/news/20210909/mu-covid-variant-qa 

Tracking SARS-CoV-2 variants. (2021, September 2). Retrieved September 11, 2021, from  https://www.who.int/en/activities/tracking-SARS-CoV-2-variants/ 

Wadman, M. (2021, September 7). United States boosts tracking of coronavirus strains as Mu variant draws scrutiny. Retrieved September 11, 2021, from https://www.science.org/content/article/united-states-boosts-tracking-coronavirus-strains-mu-variant-draws-scrutiny

Williams, K. et al. (n.d.). Antibody Titers, Retrieved September 15, from https://vcahospitals.com/know-your-pet/antibody-titres