This is why the latest booster is based on the mRNA of two variants. Both to increase your immunity to an increased number of variants (there is some cross protection from related variants, depends on how similar the spike protein/antigen is to the original that the antibodies previously produced (e.g. via vaccination).) as well as to increase your "standing army" of antibodies (specific antibodies levels drop after a while, leaving memory cells that will "activate" when reexposed to the Covid antigen (variant-specfic spike protein). Problem is the memory cell response might be too slow, hence the need for boosters of the same variant.

Flu shots are a good example of this and we will likely be moving to a similar approach with COVID.