What to Know About Mu, a New Covid Variant Detected in 39 Countries

What to Know About Mu, a New Covid Variant Detected in 39 Countries

The World Health Organization said that a variant of the virus that causes Covid-19 that is now responsible for a growing proportion of cases in Colombia, and which has been detected in 39 countries around the world, is now a “variant of interest.”

Designating the variant with the Greek letter Mu, the agency said that early data indicates that the immunity provided by vaccines or through prior infection “may not be as strong against this variant,” though more studies are needed.

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In a statement, the WHO said that Mu, also known as B.1.621, “has been designated as a Variant of Interest as it has some mutations that need to be studied for their potential impact on the body’s immune response.”

The WHO tracks variants of interest, which have the potential to be more dangerous than other forms of the virus, and variants of concern, which are already posing a more serious threat. Mu is one of five current variants of interest; there are four variants of concern, including Delta.

Mu was first identified in Colombia in January, and is now responsible for 39% of cases in that country, the WHO says. Global prevalence of the variant is low, and falling, the agency says, but in recent weeks it has grown sharply in both Colombia and in Ecuador, where it now represents 13% of sequenced cases.

In a weekly epidemiological update on Covid-19 published on Tuesday, the agency said that there have been some larger outbreaks of Mu in South America and Europe.

“The Mu variant has a constellation of mutations that indicate potential properties of immune escape,” the update said. It said that early data suggested that the capacity of blood sera from vaccinated people and people who had recovered from Covid-19 infections to neutralize the Mu variant was similar to their capacity against the Beta variant, also known as the South African variant, which is notably evasive of immune responses.

Reports of Mu have been widespread in recent months. In late July, Florida television news station WPLG reported that the variant now known as Mu was responsible for 10% of Covid-19 samples sequenced at a University of Miami lab. In early August, Reuters reported that seven fully vaccinated residents of a Belgian nursing home had died from an outbreak of B.1.621.

The U.S. Centers for Disease Control and Prevention does not yet list Mu as a variant of interest or a variant of concern, though the European Center for Disease Prevention and Control has previously listed it as a variant of interest. The ECDC, an agency of the European Union, says that there is evidence that the mutations in the variant now known as Mu has an impact on transmissibility and immunity.

Mu is the first new variant to be added to the WHO’s list of variants of interest since Lambda was added in June. Delta, the most recently added variant of concern, was upgraded to that status in early May.

“WHO will closely follow the epidemiological evolution of this variant, along with studies on its impact,” the agency said.

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