With the Delta wave cresting in the US, do we need to worry about a new variant out of Columbia? The World Health Organization has designated the Mu variant “of interest,” but thus far it appears to be no competition to Delta’s dominance:

A coronavirus variant known as “mu” or “B.1.621” was designated by the World Health Organization as a “variant of interest” earlier this week and will be monitored by the global health body as cases continue to emerge across parts of the world.

Where was it first detected and where is it now?

The variant was first detected in Colombia in January 2021, where cases continue to rise. It has since been identified in more than 39 countries, according to the WHO, among them the United States, South Korea, Japan, Ecuador, Canada and parts of Europe. …

Mu has yet to be designated a variant of interest or concern by the Centers for Disease Control and Prevention, which did not immediately respond to a request for comment from The Washington Post.

NIAIH director Anthony Fauci did offer some comment on Mu yesterday, telling reporters on a White House briefing that his team was investigating it. Despite some suggestions that the mutations on Mu might help it evade some antibodies induced by vaccinations, Fauci warned that these were only laboratory results and narrowly focused. He expects vaccines to hold up well against Mu for prevention of serious illnesses, and appears to cast doubt on its ability to compete against Delta:

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said on Thursday that the U.S. is taking the variant, dubbed Mu, “very seriously,” but that it hasn’t taken an extensive hold in this country.

“We’re keeping a very close eye on it. It is really seen here, but it is not at all even close to being dominant,” Fauci said. “As you know, the Delta is more than 99% dominant.”

Fauci said the Mu variant, technically known as B.1.621, has mutations suggesting “it would evade certain antibodies,” potentially including those from vaccines.

“But there isn’t a lot of clinical data to suggest that. It is mostly laboratory in vitro data,” he added. “…We don’t consider it an immediate threat right now.”

Scripps researcher Eric Topol dismisses the concerns more directly this morning on Twitter, linking to a threat by British epidemiologist Meaghan Kall. Topol is hardly a denialist when it comes to COVID; in another tweet this morning, he cheers Australia’s zero-COVID strategy. But the data on Mu and other variants shows that it just doesn’t compete with Delta and doesn’t appear to gain much traction in vaccinated populations:

A lot has recently been made of 3 variants: Lambda, Mu (B.1.621) and C 1.2. None of them are/can compete with Delta and will fade away. This new @PHE_uk report with @kallmemeg's thread addresses the C.1.2 story https://t.co/qv5gKIbqxt

— Eric Topol (@EricTopol) September 3, 2021

A few persistent cases of Alpha, but very nearly dying out now… pic.twitter.com/Nzw8mBB2sb

— Meaghan Kall (@kallmemeg) September 3, 2021

And our newest variant, Mu 🐮 with no new cases since 12 August pic.twitter.com/ydnrItL7Fd

— Meaghan Kall (@kallmemeg) September 3, 2021

That would suggest that relatively heavily vaccinated populations such as the UK don’t provide much opportunities for Mu to propagate. That may be especially true for countries where the Delta variant has already ripped through the unvaccinated part of the population. The effects of either natural or induced immunity appear to contain Mu much better than Delta, at least from the small amount of real-world data we have at the moment. That beats the test-tube data on antibodies, which are only part of the human immune response and tell only incomplete stories about resistance to the variants.

This is almost certainly why WHO and Fauci consider it a “variant of interest” at the moment, but not more. Delta is their primary concern, and vaccination to curtail it their primary mission. It’s definitely worth watching, but not worth sending people back into lockdowns.