> mRNA vaccines tell our cells to make a piece of the “spike protein” that is found on the surface of the SARS-CoV-2 virus. Since only part of the protein is made, it does not harm the vaccine recipient, but it is antigenic and thus stimulates the immune system to make antibodies.
Source: https://www.cdc.gov/vaccines/covid-19/hcp/mrna.html
Additionally, one of the studies referenced in the linked paper also states that negative thromboembolic events are observed in vector-based vaccines but are absent in mRNA vaccines:
> Here, we present first molecular evidence that vector-based vaccines encoding the Spike protein exhibit a problem that is completely absent in mRNA-based vaccines. This is due to the fact that during the vaccination step, the adenoviral DNA enters the nucleus and use the host machinery to transcribe its (trans)genes inside the nucleus.