every molecule has different effects. Change the wrong amino acid residue in insulin and you get something that hyperactivates IGF-1 pathway and causes cancer,

every molecule has different effects. Change the wrong amino acid residue in insulin and you get something that hyperactivates IGF-1 pathway and causes cancer, which you won't see in the short term. You can't be sure that any given vaccine won't accidentally activate an oncogene, until you do the study, and such a study would necessarily take years, or decades.