From what I read it’s a lot more complex than that. One theory is that it’s due to how the AZ vaccine is constructed. It is a neutered chimp adenovirus that can

From what I read it’s a lot more complex than that. One theory is that it’s due to how the AZ vaccine is constructed. It is a neutered chimp adenovirus that cannot reproduce but is able to infect cells and cause them to produce the COVID spike protein that the immune system can detect and react to. So it is a two stage process: infect the cell, make it produce the spike protein, have the spike protein attach to another cell, and finally have the immune system respond to the spike protein. The idea is that if you inject too high a dose initially, the immune system will instead respond to the adenovirus and kill it before it can carry out its intended job here. Having a slower initial response will allow for enough COVID spike protein to be produced to develop an immunity to that. I am absolutely not an expert on this. Just paraphrasing something I read.