As Mu Variant Spreads in U.S., This Is Why Some COVID Mutations Take Hold and Others Don't

The Mu variant of COVID has made numerous headlines since it was classified as a variant of interest (VOI) by the World Health Organization (WHO) last week, but it does not appear to be taking over the U.S. yet, data shows.

According to the most recent genomic surveillance data published by the Centers for Disease Control and Prevention (CDC), Mu, or B.1.621, accounted for just 0.2 percent of sequenced samples in the country in the week ending August 28.

In that same week, the Delta variant accounted for more than 99 percent of sequenced samples.

A few days later the WHO announced that the global prevalence of the Mu variant was less than 0.1 percent—but that in Colombia and Ecuador, prevalence was significantly higher, at 39 percent and 13 percent respectively.

There are suggestions that Mu might pose some resistance to immunity, but for now the variant is still a variant of interest rather than a variant of concern, while further study is ongoing, putting it in the same league as other variants such as Kappa (B.1.617.1), Iota (B.1.526), and Eta (B.1.525), according to the WHO and CDC.

Those variants are essentially non-existent in the U.S. when compared to Delta, according to the aforementioned CDC genomic surveillance data.

So Why Do Some Variants Take Off While Others Fizzle Out?

The answer is down to chance, according to Dr Francesca Beaudoin, interim chair of epidemiology at Brown University in Rhode Island.

"Viruses mutate or change themselves at random, all the time," she told Newsweek.

"Most mutations are inconsequential or make the virus less infectious and hence why they fizzle out. Other mutations may give the virus certain advantages that make it more likely to spread and cause illness."

Where each variant first appears also makes a difference, Beaudoin said. There's an element of randomness in this, but certain conditions, such as low vaccination rates and population density, will fuel the spread of a variant.

These changes between COVID variants also mean that they are pitted against one another in a situation that is, essentially, survival of the fittest. This is because new variants may have some advantage over the one or ones that were there before them.

"The viruses that have the biggest advantage in terms of infecting people and spreading is the one that 'wins'," Dr Davey Smith, head of the Division of Infectious Diseases and Global Public Health, University of California San Diego, told Newsweek.

"Some variants may have spread better in one population than another variant (i.e., have a fitness advantage) because populations may have different rates of immune responses from vaccination or natural infection."

Researchers now have an idea of which particular mutations seem to confer certain benefits to different variants of COVID. The notorious E484K mutation, for instance, helps COVID slip past the body's immune defences while research has shown that the N501Y mutation may make the virus more transmissible.

That said, predicting which variants might end up spreading and which will not is hard to predict based solely on the structure of its mutations, said Dr Stephen Kimmel, chair of the Department of Epidemiology at the University of Florida.

He told Newsweek: "In terms of Mu, the question is whether it will outcompete the Delta variant. As of now, that does not seem to be happening."

One thing we do know is that as long as COVID has the opportunity to spread—such as through unvaccinated populations—the more mutations will occur, Kimmel added.

COVID sampling
Medical personnel working with a COVID sample at a drive-through testing site in San Francisco, California, in March 2020. The Mu variant of COVID does not appear to be outcompeting the Delta variant in the U.S. at the moment. Justin Sullivan/Getty