The SARS-CoV-2 Mu variant emerges

Sept. 23, 2021

While Delta is the dominant SARS-CoV-2 variant, a newcomer, Mu, has gained the world’s attention.

Mu became the World Health Organization’s (WHO) fifth variant of interest (VOI) on August 30. Other variants in this category are Eta (B.1.525), Iota (B.1.526), Kappa (B.1.617.1) and Lambda (C.37).

WHO also has recognized four variants of concern (VOC), a more severe category that includes Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1) and Delta (B.1.617.2).

However, the available information on the Mu variant, or B.1.621, is preliminary and inconclusive.

It was first identified in Columbia in January 2021. Since then, “there have been a few sporadic reports of cases of the Mu variant and some larger outbreaks have been reported from other countries in South America and in Europe,” the WHO wrote recently in its COVID-19 Weekly Epidemiological Update.

As of early September, the United States reported 2,462 cases overall, including 60 in the previous four weeks. On the same date, case counts were 1,041 in Columbia, 516 in Spain, 375 in Mexico, 182 in Chile, 170 in Ecuador, 128 in Canada, and 62 in the United Kingdom, according to the Global Initiative on Sharing All Influenza Data (GISAID).

Overall, Mu has been detected in 49 countries.

The WHO said it classified Mu as a VOI because preliminary data showed a potential reduction in the neutralization capacity of vaccines and some treatments, such as convalescent plasma. In general, a variant becomes a VOI if it includes genetic changes that affect transmissibility, disease severity, immune escape, diagnostic or therapeutic escape, and if it has caused community transmission or community clusters in multiple countries with increasing prevalence over time.

As of early September, however, the Centers for Disease Control and Prevention (CDC) had not added Mu to its SARS-CoV-2 variant classification system, which includes four variants of concern (Alpha, Beta, Delta, Gamma) and four variants of interest (Eta, Iota, Kappa and Pango Lineage B.1.617.3).

At a recent news briefing, Anthony Fauci, MD, Director of the National Institute of Allergy and Infectious Diseases and Chief Medical Advisor on COVID-19 to the President, said the United States is keeping an eye on Mu, even though the variant “is not at all even close to being dominant.” At this point, Delta accounts for more than 99% of cases, Fauci explained.

Fauci also addressed concerns about whether Mu can evade antibodies, noting that most of the information about the variant so far is preliminary data from in vitro laboratory testing — not clinical data from patients’ medical histories. “Not to downplay it; we take it very seriously,” he added.

What does this information on Mu mean for clinical labs? Like other aspects of the COVID-19 pandemic, laboratorians probably should keep abreast of the latest news and research findings on Mu, just as they do for many other aspects of the rapidly evolving COVID-19 pandemic.

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