This retired neurosurgical anesthesiologist's immediate, off-the-top-of-my-head response is that you're spot-on in zeroing in on this. If lost immunity over months proves to be the case, vaccination would require constant boosters — say, every 2-3 months. If by injection, a non-starter. Orally, like the Sabin polio vaccine, that's another story. All the many current vaccination trials require IM injections. See, for example: https://www.nature.com/articles/d41586-020-01092-3