This is an excellent explanation. Also, it is believed that our body actually develops an antibody to attack the AZ vaccine carrier virus so the second shot is destroyed before it has a chance to deliver that DNA. This explains the paradox why waiting longer, such as 12 weeks, seems to increase efficacy of such vaccine when compared to 3 weeks like Pfizer. It reduces that chance of second dose destruction. To overcome this, Russian Sputnik-V vaccine (also using carrier virus) uses a different virus carrier so it is naive for your body. Therefore, their first and second doses are actually different and not interchangeable, unlike AZ vaccine. Amazing science, eh?
yes that certainly could be the case that the body is attacking the carrier virus. The Novavax vaccine that is in the news lately but won't likely be approved for at least multiple months takes it even a step further and is a direct injection of the spike protein rather than getting your body to produce it with mRNA like the mRNA vaccines do.