The World Health Organization is monitoring a new coronavirus variant called "mu," which the agency says is now labeled a "variant of interest."
What does that mean?
There are three so-called categories of variants being monitored by global and U.S. health officials, but not all variants rise to the level of such classification.
Variants are categorized as "variants of interest," "variants of concern" and "variants of high consequence."
Get Chicago local news, weather forecasts, sports and entertainment stories to your inbox. Sign up for NBC Chicago newsletters.
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 Chicago Department of Public Health Commissioner Dr. Allison 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 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.'"
Variants of interest differ between the CDC and WHO.
Most recently, Mu — also known by scientists as B.1.621 — was added to the WHO's list of variants "of interest," the international health organization said in its weekly COVID epidemiological report published late Tuesday.
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 mu strain needs further study to confirm whether it will prove to be more contagious, more deadly or more resistant to current vaccines and treatments.
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."
Currently, the most prevalent variant of concern in the U.S. is the delta variant, which now makes up a majority of COVID infections.
But it's the variant of high consequence that Arwady says she is most concerned about.
"I would say if something became a variant of high consequence that would be a big deal," she said.
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.
"New variants should not surprise anybody, but the main thing we're watching is that the vaccines continue to protect well, which they do, especially against hospitalization and death," Arwady said.