People line up every year to get a new flu shot because seasonal influenza keeps mutating the part of its hemagglutinin protein where most neutralizing antibodies bind after exposure to a vaccine that carries the entire hemagglutinin protein (embedded in a weakened/inactivated influenza virus).

Sure, spike ain't hemagglutinin and corona ain't no influenza, but let's not pretend there's no precedent at all for proteins mutating to the point that vaccines become ineffective.