"a protein—or even just a piece of a protein—that triggers an immune response inside our bodies."
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/different-vaccines/mrna.html
"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