Spike protein is the part of the Whole virus that the Immune System identifies and then eliminates via antibodies , and so enables you to overcome infection. So in order to produce antibodies only the Spike protein is needed, hence why the jab only contains the mRNA required for the body to create Spike Protein - which in turn stimulates the immune response to create antibodies, and in doing gives you protection.
This protection wanes as the Virus changes which is why the original jabs developed against the Alpha are far less potent against the Omicron strain.
