Mu Variant: A Look at Cases Reported in Illinois and What We Know So Far

NBCUniversal Media, LLC

The World Health Organization is monitoring a new coronavirus variant called "mu," which the agency says is now labeled a "variant of interest," and although the variant is not yet on the list being monitored by U.S. health officials, cases have been reported in Illinois.

According to, a Scripps Research project tracking COVID-19 genomic data such as lineages and mutations, as many as 21 cases have been identified in Illinois between the months of April and August, representing what is estimated to be less than 0.5% of cases in the state.

The group notes, however, that because its data is not a random sampling of mutations, it "does not indicate the true prevalence of the mutations but rather our best estimate now."

Federal health officials in the U.S. stressed last week that the "mu" variant is not an immediate threat to the United States.

"We're paying attention to it, we take everything like that seriously, but we don't consider it an immediate threat right now," White House chief medical advisor Dr. Anthony Fauci said during a press briefing Thursday.

Chicago Department of Public Health Commissioner Dr. Allison Arwady also noted the variant has not yet been categorized as a variant of interest from the U.S. Centers for Disease Control and Prevention.

"So actually the CDC has not even declared mu a variant of interest at this point because it's not been behaving in a way that's been particularly concerning," she said during a Facebook Live Tuesday. "It's not been out competing delta, it's not been making people super sick. It doesn't mean it couldn't emerge, but right now it's not even a variant of interest in the U.S."

Still, mu — also known as B.1.621 — has been added to the WHO's list of "variants of interest," the international health organization said in its weekly COVID epidemiological report published last week.

The variant contains genetic mutations that indicate natural immunity, current vaccines or monoclonal antibody treatments may not work as well against it as they do against the original ancestral virus, the WHO said. The strain needs further study to confirm whether it will prove to be more contagious, more deadly or more resistant to current vaccines and treatments.

"This variant has a constellation of mutations that suggests that it would evade certain antibodies, not only monoclonal antibodies, but vaccine- and convalescent serum-induced antibodies," Fauci said. "But there isn't a lot of clinical data to suggest that, it is mostly laboratory in-vitro data."

The World Health Organization is monitoring a new coronavirus variant called “mu,” which the agency says is now labeled a “variant of interest,” and although the variant is not yet on the list being monitored by U.S. health officials, cases have been reported in Illinois.

Variants are categorized as "variants of interest," "variants of concern" and "variants of high consequence" as they are monitored by global and U.S. health officials.

Variants of interest differ between the CDC and WHO.

The CDC states that a variant is classified as a variant of interest if it shows "specific genetic markers that have been associated with changes to receptor binding, reduced neutralization by antibodies generated against previous infection or vaccination, reduced efficacy of treatments, potential diagnostic impact, or predicted increase in transmissibility or disease severity."

According to Arwady, this category is most common.

"We will probably have a variant of what we call a variant of interest identified every few weeks," Arwady said last week. "Like that's how this virus works. Variants of interest just mean we've seen a number of the same genetic mutations emerge and scientists around the world are on the lookout for it every single time. A virus anywhere in the world gets this genetic sequence, it gets updated into a database that's shared internationally, so we can really see what's emerging. When a variant of interest emerges, it says, 'Oh, this is something we should watch a little bit.'"

So what must happen for a variant of interest to become a variant of concern?

A variant of concern is one in which "there is evidence of an increase in transmissibility, more severe disease (e.g., increased hospitalizations or deaths), significant reduction in neutralization by antibodies generated during previous infection or vaccination, reduced effectiveness of treatments or vaccines, or diagnostic detection failures," according to the CDC.

"Once we see a variant of interest clearly have an impact, meaning it is more contagious, it might be evading a treatment, it's clearly making people sicker - that makes it into a variant of concern," Arwady said.

Currently, there are four variants of concern in the U.S., including Alpha, Beta, Delta and Gamma.

Finally, a variant of high consequence "has clear evidence that prevention measures or medical countermeasures have significantly reduced effectiveness relative to previously circulating variants." Or, as Chicago's top doctor put it, a variant in which "the vaccine was not working very well."

There are no such variants reported, however.

Currently, the most prevalent variant of concern in the U.S. is the delta variant, which now makes up a majority of COVID infections.

According to the Centers for Disease Control and Prevention, variants of the coronavirus were expected.

"Viruses constantly change through mutation, and new variants of a virus are expected to occur," the CDC's website reads. "Sometimes new variants emerge and disappear. Other times, new variants persist. Numerous variants of the virus that causes COVID-19 are being tracked in the United States and globally during this pandemic."

As variants continue to spread, the agency suggests that vaccinations and following health protocols are the best way to slow the spread. And while all COVID-19 tests can detect different variants, they will not tell you which variant you have.

"Remember, even when you have variants that do diminish somewhat the efficacy of vaccines, the vaccines still are quite effective against variants of that time," Fauci said.

Contact Us