By Aristos Georgiou

The Mu COVID-19 variant has been garnering increased attention over the past few weeks and was recently designated as a "variant of interest" by the World Health Organization (WHO). But is Mu more dangerous than the Delta variant that is fueling the latest wave of infections in the United States?

The Delta variant has spread to more than 170 countries around the world since first being identified in India in October 2020, becoming dominant in many regions.

In the United States, for example, Delta is now totally dominant, accounting for more than 99 percent of new COVID-19 cases, figures from the Centers for Disease Control and Prevention (CDC) show.

According to the CDC, the Delta variant causes more infections and spreads faster than earlier forms of SARS-CoV-2—the virus that causes COVID-19.

Studies show that Delta is highly contagious—perhaps more than twice as contagious as previous variants.

There is also some evidence to suggest that Delta might cause more severe illness than previous variants in unvaccinated people.

Some research has shown that individuals infected with Delta, which has been listed as a "variant of concern" by the CDC, appear to have viral loads 1,000 times higher than those seen with previous variants.

While COVID-19 vaccines are effective at reducing infections with Delta—as well as severe disease—breakthrough cases can still occur. Fully vaccinated people with breakthrough infections can still spread the virus to others, although they appear to be infectious for a shorter time.

Delta has multiple mutations on the spike protein of the virus. This particular set of mutations make the variant much more effective when it comes to binding and entering human cells, hence why it is so contagious.

Much less is known about the Mu variant, which the WHO added to its "variant of interest" (VOI) list on August 30.

The variant, which was first identified in Colombia in January 2021, has now been confirmed in more than 40 countries around the world and all 50 U.S. states.

The WHO designated Mu as a VOI due to significant outbreaks in South American countries such as Colombia and Ecuador, as well as some parts of Europe.

But in the U.S., Mu accounted for only around 0.1 percent of new infections in the week ending September 11, according to the CDC. And the proportion of new cases caused by the new variant has been falling since July as Delta has become more dominant. The CDC has not yet listed Mu as a VOI.

The Mu variant has several mutations in the spike protein, with initial research indicating that it could be more resistant to vaccines or natural immunity than earlier variants (although the full extent of this is not yet clear).

But WHO officials have said that the Delta variant is a far more pressing concern due to its highly contagious nature.

Maria Van Kerkhove, the agency's technical lead for COVID-19, said during a virtual press conference last Tuesday: "The Delta variant for me is the one that's most concerning because of the increased transmissibility."

"In some countries, the proportion of cases with the Mu variant is increasing," Van Kerkhove said. "But in other countries, the proportion of Mu is decreasing. Where Delta is, Delta takes over really quickly."

Dr. Mike Ryan, head of the WHO's health emergencies program, said at the press conference that any new variant has to compete with the "best of class," which is currently Delta. This variant tends to "outcompete" other variants, he said, even if they are better at evading the protection afforded by vaccines or natural immunity.

Earlier in September, White House chief medical adviser Dr. Anthony Fauci, said health authorities in the U.S. were monitoring Mu closely while downplaying the threat posed by the variant.

"We're paying attention to it, we take everything like that seriously, but we don't consider it an immediate threat right now," Fauci said at a press briefing on September 2.

John P. Moore, a professor of microbiology and immunology at Weill Cornell Medical College, previously said that while Mu has some "troubling mutations," it is not spreading widely in the United States.

"I've seen very few studies on its properties because it's not been widely studied yet. The key thing is, it's just not spreading much."

The fact that Mu isn't spreading as rapidly as Delta could mean that the new variant will gradually disappear, according to Moore.

"One of the things we've learned with Delta is that it squelched out less transmissible variants," Moore said. "It's the most transmissible variants that dominate. And everything else just becomes less and less of an issue. Unless Mu has unique transmissibility properties, and that would be surprising because it's not been seen so far. It's survival of the fittest from the virus perspective."

And even if Mu turns out to be more resistant to vaccines than other strains, experts say the shots that are currently available will likely still be effective at protecting against severe disease.

Jesse Erasmus, PhD, director of Virology at HDT Bio, and acting assistant professor at the University of Washington School of Medicine in Seattle said: "A preprint study (not yet peer-reviewed) came out September 7, and indeed demonstrates that Mu is slightly more vaccine resistant than the Beta variant."

"When we talk about vaccine escape, we have to be very careful to specify this is escape from infection, not from serious disease. Even against the most vaccine resistant variant out there, the vaccines still protect against disease."