Dr. John Campbell had a guest on recently that discussed the similarities between allergy treatment and getting boosters with the same ingredients over and over. He said that when you treat an allergy you give small doses of the allergen to the patient over a time period and they desensitize the patient to that allergen. He said that it could be possible that when you give the same "vaccine" over and over again, you are making the patient become desensitized to the ingredients of the booster and thus end up having negative efficacy to the treatment.
So to me, it looks like one vaccine sensitizes you to the disease and it makes you fight it better and many boosters de-sensitizes you to the disease and your body cannot fight it off any longer.