What is the Mu variant of the coronavirus?

An illustration of a coronavirus with a question mark inside
Scientists are still learning about the Mu variant of the coronavirus, which accounts for about 0.1% of infections in the United States.
(Peter Hamlin / Associated Press)

The Delta variant may be dominating the coronavirus landscape, but the lesser-known Mu variant is starting to attract more attention.

This “variant of interest” has been detected in 167 people in Los Angeles County over the summer, officials said this week.

What do scientists know about the Mu variant?

It’s a version of the coronavirus that was first identified in Colombia in January. Since then, it has caused isolated outbreaks in South America, Europe and the United States.

The World Health Organization designated Mu a “variant of interest” last month because of concerns it might make vaccines and treatments less effective — though more evidence is needed to know whether this is indeed the case. If it turns out to have attributes such as an ability to spread more efficiently or to make infected people sicker, it could be upgraded to a “variant of concern.”

Scientists 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 simply fade away.


The Biden administration is boosting efforts to identify and track coronavirus variants to help scientists see where the pandemic is heading next.

So far, the Mu variant doesn’t seem to be spreading quickly: It accounts for fewer than 1% of COVID-19 cases globally. In Colombia, it may be responsible for about 39% of cases.

Most countries remain concerned about the highly contagious Delta variant, which now dominates in all of the 174 countries where it’s been detected. It’s estimated to account for more than 99% of all coronavirus infections in the U.S., while Mu makes up 0.1%, according to the Centers for Disease Control and Prevention.

The U.S. is “paying attention to it,” but it isn’t considered an immediate threat, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

Officials have been tracking the Mu variant in Europe, where it has been seen in about a dozen countries. According to a recent statement from the French Ministry of Health, Mu “does not seem to have increased recently” across Europe.

A new coronavirus variant has emerged from South Africa. It has a high number of mutations but has a ways to go to challenge Delta.

A report from England’s public health agency last month suggested the Mu variant might be as resistant to vaccines as the worrisome Beta variant first seen in South Africa but added that more real-world data were needed.

WHO officials said the Mu variant appeared to be rising in some countries in South America but that the Delta variant still was spreading far more easily.

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