[erhk]

What does region mean in this context? A region of the body?

Antigen binding?

Your statement on future infections by a distinct antigen than you were immunized for is objectively true. A vaccine is not a panacea for all viruses.

If I prepare my home for a flood then I shouldn't expect an earthquake to be stopped.

If we are making the case that a surface protein is too targetted then I would point towards the seasonality of the flu.

[somewhereoutth]

Region of the virus - e.g. capsid, spike etc. Even within a variant such as Delta there are sub-lineages with slight differences. Most diffences make no difference to the transmissibility etc of the virus, so can arise (and disappear) fairly randomly. If your immune system preferentially produces antibodies to proteins on the virus encoded by such a difference, then the next COVID virus it encounters may not have these proteins, and so your antibodies won't bind.