It's because of the way the AZ vaccine works. It uses a different virus to infect cells which then create the antigen to stimulate the immune response. These ar

It's because of the way the AZ vaccine works. It uses a different virus to infect cells which then create the antigen to stimulate the immune response. These are a bit unstable, since the vaccine itself, being a virus, is an immune system trigger. The immune system might snuff out the vaccine itself after a huge first dose, then the second dose is wiped out. This is I think why the mRNA vaccines are doing better. They don't use a virus as a vector, they use a less convoluted vector of a more basic mRNA instruction set. They're also in a way more "scalable" in that a future disease or a mutation of Covid with a new spike protein could have a new vaccine made quickly by tweaking the mRNA sequence.