The same will hold true for people vaccinated with 2 doses who experience vaccine failure.

Make the population immune without eradication/elimination, and the selective pressure for immune escape is intense.

— Kelly Wind (@kellywind) June 20, 2021

What Kelly says is not wrong, exactly, but it is misleading because it misses vast amounts of complexity and caveats. Certainly, SARS2 having more hosts in which to evolve is not ideal. But the virus, as with all of us, neither has infinite flexibility nor infinite capability to achieve optimum configurations. There are many superior protein arrangements that we can imagine, and many that we might be able to artificially place it (and similar viruses) into that it can never achieve via evolution.

Explaining exactly why would take far too long and most of you would need to read quite a few textbooks before that, but here’s a diagram that relates:

This is much, much simplified, but the virus started at 1, which was metastable (from our perspective). The variants will tend to take it to 3, a stable equilibrium. To go beyond that, there is a much, much harder hill to climb to reach beyond 3, and no natural evolutionary process can do so for the most-optimum configurations.

I think there’s only about a 50% chance I’ll be right, but I expect the Delta variant to be the last or the next-to-last variant to emerge that is significantly more harmful than others. With the number of hosts infected, and the limited configuration space for ACE2 receptor binding, I expect SARS2 is about out of tricks.

As ever, though, time will tell the full tale.