Unlike the Pfizer-BioNTech and Moderna vaccines, the Johnson & Johnson vaccine does not use messenger RNA (mRNA) to help the body build its defenses against the virus. Instead, it is an andenovector vaccine. The scientists behind the Johnson & Johnson vaccine added the gene for the coronavirus’ signature spike protein to an adenovirus, a common virus that causes colds or flu-like symptoms. However, the adenovirus was modified so it can enter cells but can’t replicate or cause illness.
J&J uses an adenovirus to deliver DNA into the cell nucleus, causing it to produce mRNA which codes for the spike protein just like the other vaccines. They all work basically the same way. It’s just that J&J uses a slightly different route to get there.