If you want to sound stupid you can give the definition of Immunity for legal process, or, you can use the definition used in medical science which is;
the ability of an organism to resist a particular infection or toxin by the action of specific antibodies or sensitized white blood cells.
Does not mean complete protection, just the ability to form resistance to infection or toxin. So no vaccination ever guarantees full protection, however in Covids case most vaccinations give between 60-80% complete resistance to the virus. You are also given resistance enough that any breakthrough is 97% less severe, and half the duration.