While the mu variant of the coronavirus has continued to slowly spread throughout the world over the last several months — including in 49 U.S. states — local and state health officials say Washingtonians shouldn’t be too concerned at this point.

The variant was first identified in Colombia in January and has since caused isolated outbreaks in South America, Europe and the United States. Last month, the World Health Organization listed mu as a “variant of interest” because of concerns that it could make vaccines and treatments less effective. However, more evidence is needed, the WHO said.

In Washington, the variant was first detected in April. A month later, King County public health officials also confirmed they’d identified the variant.

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Since then, King County has counted 39 total mu cases, fewer than four cases per week, county public health officer Dr. Jeff Duchin said Wednesday.

“There’s no indication for an increasing trend,” Duchin said. “Mu looks very bad in a test tube, and it might have the potential to evade vaccine-based immunity, but whether a virus has any real human health significance depends on a lot more than how it looks in a test tube.”


A virus must be able to survive in the “real world” and outcompete other variants, such as the delta variant, in order to have a large impact on community health, he added.

“The course of the outbreak has been fraught with uncertainty and it will continue to be. … We should expect to see an ongoing evolution of viruses that may appear to have potential concerning features, many of which will never pan out to be significant human health concerns,” Duchin said.

He added, “It’s very important not to assume anything. … We should not be overly concerned about mu at this point.”

Coronavirus cases involving the mu variant also haven’t increased much statewide, though experts are continuing to monitor its spread, according to Teresa McCallion, spokesperson for the state Department of Health.

In August, there were 3,442 specimens sequenced from Washington residents, with mu representing about 0.4% of sequenced cases — compared to 98.2% of cases represented by the delta variant, McCallion said.

More data is needed to better analyze the variant, but Duchin said Wednesday mu may “never become a contender” among other coronavirus variants.


Globally, the delta variant remains more dominant in almost all of the 174 countries where it’s been detected. According to The Associated Press, mu accounts for fewer than 1% of coronavirus cases throughout the world, though it could be responsible for about 39% of cases in Colombia.

Scientists generally monitor emerging coronavirus variants based on suspicious genetic changes and then look for evidence to determine whether the new version is more infectious or causes more severe illness. Viruses evolve constantly and many new variants often fade away.

The mu variant “is of interest to us because of the combination of mutations it has,” said the WHO’s Dr. Maria Van Kerkhove. “But it doesn’t seem to be circulating.”


Material from The Associated Press and New York Daily News was used in this story.