Modified RNA vaccines look for "parts" of the virus, not the virus itself. While they will lessen the severity if you do become infected, once the antibodies are gone its the same as never having seen the virus, as you body did not have to heal and create B and T cells in your bone marrow. The antibodies were triggered by a part of a virus and not the virus, in this case, the "spike proteins". You gain no long term protections.
This is also why you can become infectious to others. The virus grows unabated until there are sufficient "spike proteins" to trigger a response from your immune system. This is why people get ill, become infective, but have very mild cases. It is the lack of healing and the trigger by spike proteins which stops the body from creating long term immunities in B and T cells. Once your antibodies are gone from this mRNA vaccine your body has never seen or reacted to the actual virus, you are now a walking time bomb again to have a very sever case.