The highly contagious delta variant has utterly transformed the COVID-19 pandemic. It’s also a major reason why the United States reached a grim new milestone this week: There are now more than 40 million known coronavirus cases, according to The New York Times.
But delta isn’t the only variant officials are keeping an eye on. There has been an interest recently in the “mu” variant — also known as B.1.621 — which is steadily increasing in certain parts of the world, and has been identified here in the U.S. in 49 states. Here’s what you need to know.
Where did experts discover it and where has it spread?
The variant was first documented in Colombia in January 2021 and it has been found in 39 countries to date, per the World Health Organization, which identified mu as a “variant of interest” last week.
But from a global perspective, it’s still relatively rare.
The mu variant makes up less than 0.1% of sequenced COVID-19 cases around the world — although its prevalence in some countries has been consistently increasing. Mu makes up 39% of cases that have been subject to genomic sequencing in Colombia and 13% in Ecuador.
Here in the U.S., mu makes up just 0.2% of sequenced cases, or about 2,400 cases total. But it appears to be clustered in some spots. For example, Los Angeles County health officials say it’s been detected in 167 people in that area over the summer.
Why is mu a variant of interest?
The WHO considers a new mutation a “variant of interest” if it has genetic changes that affect the virus’s characteristics (like how transmissible it is, or whether it’s more likely to cause severe disease). It can also be a variant of interest if it has led to significant community transmission or specific clusters.
At this point, the WHO says that mu hasn’t risen to the level of being a “variant of concern,” like delta, but the agency is closely monitoring the strain because of its steady rise in countries like Colombia. The agency also notes that it’s difficult to know exactly how common it is, because there are significant variations in how much testing and genomic sequencing is being done around the world — and how quickly those results get reported.
And health experts here in the U.S. haven’t yet sounded the alarm. The Centers for Disease Control and Prevention has its own list of variants of interest and variants of concern, and mu isn’t on either.
In a press briefing last week, Dr. Anthony Fauci, the nation’s top infectious disease expert, said: “We’re paying attention to it. We take everything like that seriously. But we don’t consider it an immediate threat right now.”
How does mu stack up against delta?
The delta variant continues to dominate.
U.S. health officials say that more than 99% of the cases being sequenced here are the delta variant. Delta is more than two times more contagious than any of the strains that came before it, the CDC says. And there is growing evidence it makes people sicker, too.
So while public health experts and organizations are certainly paying attention to mu and other variants as they arise, delta continues to be their top concern.
Are people with COVID-19 immunity still protected against mu?
“The Mu variant has a constellation of mutations that indicate potential properties of immune escape,” the WHO has warned.
The group points to some preliminary data that suggest the mu variant is capable of evading certain antibodies, although it cautions that preliminary data needs to be replicated before anyone jumps to any conclusions about mu overpowering our current vaccines. Fauci recently echoed that sentiment, noting that the data that suggests mu might pose a threat to vaccine efficacy is “mostly laboratory, in vitro data.”
On the whole, the Moderna, Pfizer and Johnson & Johnson vaccines continue to hold up well against all of the various variants health officials are tracking. The New York Times recently reported that the chance of breakthrough cases among people who are fully vaccinated are about 1 in 5,000.
The vaccines also continue to serve their primary function well, which is preventing serious illness and death. The vast majority of people who are hospitalized or dying as a result of COVID-19 are unvaccinated.
How can I protect myself against mu?
Getting vaccinated remains the most important thing people can do to protect themselves against the mu variant, and it also helps protect those who are not eligible for vaccination yet, like children.
It also remains critically important to follow the most recent public health guidance about preventive measures such as wearing masks in indoor public settings — even if you’re fully vaccinated and in an area of substantial transmission. That is the vast majority of the country right now. (You can check county-level transmission here.)
Also, always wear masks on public transportation, the CDC says. Masks should fully cover your nose and mouth and fit snugly against the sides of your face.
Social distancing continues to be important as well. In public, try and put six feet of distance between you and others, the CDC says, and avoid crowds and poorly ventilated spaces. Restaurants, bars, fitness centers and movie theaters continue to put people at higher risk for COVID-19.
Experts are still learning about COVID-19. The information in this story is what was known or available as of publication, but guidance can change as scientists discover more about the virus. Please check the Centers for Disease Control and Prevention for the most updated recommendations.