"often the antigen to which the immune system responds is a relatively small number of amino acids or peptide.
A possible alternative approach to immunization would therefore be to identify the peptide sequences that trigger a protective immune response and to use completely synthetic versions of these as the vaccine substance"
https://www.who.int/teams/health-product-policy-and-standards/standards-and-specifications/vaccine-standardization/synthetic-peptide-vaccines
"For COVID-19 vaccines, all of the approved vaccines so far used the spike protein. The spike protein is located on the outside of a coronavirus and is how SARS-CoV-2 (the coronavirus) enters human cells."
https://www.nebraskamed.com/COVID/where-mrna-vaccines-and-spike-proteins-go
"While the spike glycoprotein is the main antigenic target for vaccine development, the nucleocapsid protein, a multifunctional protein that primarily functions for binding to the viral RNA genome and packing it into a long helical nucleocapsid structure, has also been found to be highly immunogenic and expressed abundantly during coronavirus infections [36]. Additionally, the nucleocapsid gene is more highly conserved and stable than the spike gene [37], [38]."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8302847/
"structural proteins from SARS-COV-2, including the spike (S), envelope (E), membrane (M), and nucleocapsid (N) proteins, were selected as target antigens for epitope prediction"
https://pubmed.ncbi.nlm.nih.gov/33895459/