Concerns over AstraZeneca vaccine How it works It starts with the delivery of the gene for the Sars-CoV-2 spike protein. In vector-based vaccines, this is delivered via an adenoviral system, the scientists write. The SARS-CoV-2 spike gene is then transcribed inside of the nucleus and subsequently exported as mRNA out of the nucleus. Arriving in the cytosol, the mRNA is again translated into the Spike protein. "And exactly here lies the problem," the scientists write, "the viral piece of DNA — deriving from an RNA virus — is not optimized to be transcribed inside of the nucleus." In the nucleus, splicing of the spike protein can occur at splice sites.