Antibodies are not long term, ever. They always degrade in the blood system. ...but that's also not the only type of immune response that a vaccine generates.

Antibodies are not long term, ever. They always degrade in the blood system. ...but that's also not the only type of immune response that a vaccine generates. Vaccines cause the creation of b-cells, which retain the memory of this virus antigen and will re-create antibodies if the virus infects the host. b-cell immunity is persistent - for years, and probably decades. But because it is very hard to measure (it resides in your bone marrow), it is therefor not often reported on. Having said that, it is hard to imagine that a 4th booster significantly increases the number of anti-covid b-cells - let alone ones that can impart immunity against the newer variants.