The Mu variant was first identified in Columbia in January, so it’s been around for 8 or 9 months. Any idea why is it getting more attention now?

Piedra: Like with any new variant, once you see it identified or spreading around parts of the country, it’s something to take notice of. One is seeing Mu, not in any high numbers or high percentages, but we are detecting it in different places throughout the United States. That makes it of interest.

According to the CDC website that tracks the proportion of variants in the United States, Delta still dominates, while the Mu variant accounts for only about 0.1% of cases. Any variant can become a variant of concern, but why do you think Mu is getting attention at the moment?…

Erasmus: What is most likely driving the increased interest is the idea that we are looking at mutations associated with variants that we already know about, like Delta. It looks like Mu has mutations already in Delta but it also has mutations from Alpha or B.1.1.7, which was also known to be highly transmissible.

There’s this idea that if you take a mutation from one variant and combine it with a mutation from another, you get a ‘super mutant virus’ that is going to wreak havoc. When in fact that is not necessarily the case.

I would say at this point – wisely so – it’s a variant of interest.

On the other hand Mu has been identified in over 40 countries and 49 states – every one except Nebraska. Does that say something about transmissibility?

Erasmus: That would depend. But these variants can arise via two different pathways. One is a single source of a new variant that gets transmitted globally – and it can be traced to a single, emergent event. In my view that is very unlikely.

The other scenario is ‘convergence evolution,’ when the variant emerges in multiple, separate instances. They perhaps provide some fitness to the virus. There can be multiple pathways through which a virus can arrive at this combination of mutations, which is probably more likely.

Also, if it does have an enhanced rate of transmission, we would expect to see a higher proportion of cases relative to the Delta variant.

Piedra: I would say at this time it’s way too early. We really need to focus now on getting over this fourth wave of COVID, which is really Delta-driven. It will be important, as this slows down or this wave comes to an end, to see which variant will show up next.