As the delta variant continues to spread in the United States, a new variant, called mu, has been deemed a "variant of interest" by the World Health Organization (WHO)—and preliminary data suggests that prior infection and vaccination may not provide strong immunity. 

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What is the mu variant?

According to WHO, the mu variant, or B.1.621, was first detected in January 2021 in Colombia.

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Since its identification, the mu variant has been detected in 42 different countries. While transmission of the mu variant is declining globally—accounting for less than 0.1% of current infections worldwide—it accounts for a significant portion of Covid-19 cases in Colombia (39%) and Ecuador (13%).

The mu variant has also been spotted in 49 states in the United States—and recently infected 167 people in Los Angeles County—but it has not yet been labeled a variant of interest by CDC. According to Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical advisor to the White House, the United States is monitoring the new variant, but it remains relatively uncommon in the nation, where delta currently accounts for 99% of all Covid-19 cases.

In its weekly epidemiological report, WHO stated that data presented to WHO's Virus Evolution Working Group show the mu "variant has a constellation of mutations that indicate potential properties of immune escape," including preliminary data showing "a reduction in neutralization capacity of convalescent and vaccine sera similar to that seen for the [b]eta variant."

While further studies are needed to confirm the initial data, WHO categorized the mu variant as a "variant of interest" due to its potential impact on the body's immune response and its significant community transmission.  Fauci reiterated the point, noting that while mu "has a constellation of mutations that suggests it would evade certain antibodies," there's a need for further clinical data.

New variants are to be expected

According to Advisory Board's Rachel Woods, it's important to acknowledge that new coronavirus variants are to be expected—but the fact that a variant is "new" doesn't mean that it's "worse," or even different from prior variants in any clinically significant way.

Woods suggests asking additional questions as evidence emerges on new variants, including: How transmissible is the new variant? Does it commonly lead to severe infection, hospitalization, and/or death? And perhaps most importantly, do current vaccines protect against serious illness caused by this variant?

Fauci added, "Bottom line, we are paying attention to it. We take everything like that seriously, but we don't consider it an immediate threat right now." (McCarthy, New York Times, 9/2; Saric, Axios, 9/1; Choi, The Hill, 9/1; Lin, Los Angeles Times, 9/6; Rivas, Fox News, 9/7)