I don’t know if they mention this because it’s a pretty long discussion but variants occur all the time. Some variants catch hold and multiply, it doesn’t even have to be because of a natural selection advantage, it might just be good luck and that the variant(s) encountered ideal spreading conditions. The variants we should be worried about are the ones that have mutations in the e protein in very specific domains of attachment.
I will listen to this later, I hope they have some information on the other proteins and how possible mutations in those could cause problems, or why they might not be a worry at all.