Delta is a mutated Covid 19. Covid 19 has many dozens of mutated versions, but most of these have been out-competed by the more effectively spreading mutations, i.e. Delta. So far, the vaccines confer immunity that is effective against Delta and the other variants.

While the vaccines were developed to act against the original Covid 19 spike protein, the immune system develops receptors to attach to various parts of the spike protein. Thus, mutations in the spike protein reduce the effectiveness of the immune response, but enough immunity remains to quickly recognize incoming virus and mount a defense.

New vaccines can be developed against the variants, but as long as the virus is mutating, it is a moving target.