The Pfizer vaccine, BNT162b2, and the Moderna vaccine, mRNA-1273, are both lipid nanoparticles containing a 4284-nucleotide-long mRNA segment encoding for the full-length spike protein of the original SARS-CoV-2 strain, with a only few differences designed to optimize translation.  The Janssen vaccine, Ad26.COV2.S, uses a non-replicating adenovirus containing a DNA plasmid to achieve the same result—an unmutated, full length spike protein sitting on the membrane of infected cells.