Are there plans to re-introduce prior variants or new, more lethal, variants that the existing vaccines could block?

Both things could happen, but this is not certain as yet. There is always ongoing research to have things on the shelf, so to speak, for use at an appropriate time to fit into the larger plans as an adjunct. So this will mean that uncertainty is inevitable because you are not dealing with natural cycles, as science presumes, but an ongoing biowar, and it is the extraterrestrials who will decide what happens next, not nature.