(WAND) - Global health officials are keeping track of the "mu" COVID-19 variant and have labeled it a "variant of interest." 

There have been cases of this variant, also known as B.1.621, recorded in Illinois, NBC Chicago reports. The station referenced outbreak.info, a Scripps Research project, which showed as many as 21 cases in the state between April and August. This represents less than 0.5 percent of Illinois cases. 

The "mu" variant is not yet on the list of variants monitored by health officials in the United States. 

The WHO has said the variant has genetic mutations indicating natural immunity, current vaccines or monoclonal antibody treatments may not be as effective against it. Further study is needed to figure out if is more contagious, more deadly or has greater resistance to current vaccines. 

Dr. Anthony Fauci, the leading expert on infectious disease in the U.S., said the variant has "a constellation of mutations that suggest that it would evade certain antibodies, not only monoclonal antibodies, but vaccine and convalescent serum-induced antibodies." There is not much clinical data suggesting this, he noted, and mostly "laboratory in-vitro data." 

Global and American health officials define variants as "variants of interest," "variants of concern" and "variants of high consequence." A variant of interest, according to the Centers for Disease Control and Prevention, is one that 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." 

The "variant of interest" category, which the "mu" variant was placed in, is the most common, Chicago Department of Public Health Commissioner Dr. Allison Arwady told the station. 

"We will probably have a variant of what we call a variant of interest identified every few weeks," Arwady said Tuesday. "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.'"

The U.S. has four COVID-19 variants of concern, including Alpha, Beta, Delta and Gamma. Delta makes up the majority of American COVID-19 cases and is the most prevalent as of early September.

A variant of concern is one where "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," the CDC said. 

The U.S. has not report any variants of high consequence. To be in this category, there has to be clear evidence the variant significantly reduces effectiveness of prevention measures or medical countermeasures relative to variants that have previously circulated.Â