"Most existing vaccines are prepared from an attenuated version of the pathogen or from inactivated disease-causing organisms or a suitable part of it e.g. a toxin. However, 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
"All of the COVID-19 vaccines currently approved in the United States are designed to instruct human cells to make harmless spike proteins — mimicking a viral protein that’s used by the SARS-CoV-2 virus to enter cells."
https://www.factcheck.org/2021/07/scicheck-covid-19-vaccine-generated-spike-protein-is-safe-contrary-to-viral-claims/
"SARS-CoV-2, which causes the coronavirus disease 2019 (COVID-19), has evolved over the past two years, acquiring many mutations mostly in its S protein, which is implicated in host cell entry, and emerged as new variants with enhanced characteristics."
https://www.news-medical.net/news/20220127/SARS-CoV-2-mutations-and-their-impact-on-spike-protein-specific-T-lymphocytes.aspx
"Sequences of the spike proteins were collected from a protein database"
https://pubmed.ncbi.nlm.nih.gov/25187696/