There’s a phenomenon in immunology which humans have known about for a number of decades, the Hoskins effect (or original antigenic sin, depending on how Biblical you like your terminology). Essentially the first version of a virus someone is exposed to is generally the only one that their immune system remembers during subsequent infections, no matter how many infections they have acquired subsequently. This is why someone’s first influenza exposure can dictate how they handle every subsequent influenza season to a greater extent than other factor including immunization via flu vaccines.

This is a problem with relying on "natural immunity" and repeatedly getting exposed to covid, also. Not just a potential problem with vaccinations.