Memory cells do not prevent infection, they live in the lymphatic system and are only activated if a pathogen gets past the "innate immune system" and gets to t

Memory cells do not prevent infection, they live in the lymphatic system and are only activated if a pathogen gets past the "innate immune system" and gets to the point we typically refer to as "infection". So, to re-use your phrase: if that were true we'd be seeing vaccinated people getting infected but recovering quickly and generally not dying, which is exactly what we see.