Originally Posted by Mesabah

The variants haven't actually escaped the antibodies, for example, Delta requires higher antibody levels for neutralization due to its greater binding ability. The antibodies basically work by unfolding the spike protein, not allowing ace2 site binding. Antibodies made for this are from the full antigen, locked in its prefusion form by a proline substitution mutation. The vaccine encodes the entire spike antigen to express via antigen presenting cells. This is a safety feature, otherwise you would get sub-optimal antibodies that enhance inflammation.

Every virus has an evolutionary trick to fool the immune system, Covid's trick is it blocks its spike antigen express to T-cells. This is devastating to the vaccine, as it can't confer long term immunity without antibody levels artificially stimulated above neutralization. If variants do finally escape neutralization from antibodies(Covid is on that evolutionary path, probably due to the vaccine schedule), then a completely new antigen is needed for the vaccine, requiring the primer/booster dose to achieve neutralization levels.

(The most effective vaccine, is the one with the highest dose, i.e. Moderna)