I wonder if this is really the case. It seems like the original vaccines trained everyone's immune system on the spike protein specifically because this is exposed and highly conserved by the virus, such that it's hard to change and still leave an effective virus. Delta, however, found such a mutation that made it more virulent and it escaped.

If we vaccinate against the modified spike, it's not clear how many more mutations exist like Delta. Maybe we can still get ahead of it, but I don't think anyone knows yet.

I do wonder if we'll see reformulated boosters against Delta's spikes, though.