The comment above by verisimi was factually incorrect. The Pfizer vaccine contains MRNA. MRNAs are short-lived nucleic acids (not DNA) that exert their function in the cytoplasm of the cell (not nucleus, where DNA lives and Cas9 exerts its' action) and are used to translate proteins. The proteins coded for by the MRNA sequences in vaccines (Pfizer, Moderna) code for the S1 spike protein, which occurs on the surface of the SARS-CoV-2. After their translation, these proteins are then recognized as foreign by the immune system of the host (i.e. person), leading to an immune response. There is no Cas9 enzyme in MRNA vaccines.