The AstraZeneca vaccine puts the spike protein onto a chimp cold virus.
My understanding is that the hard part is to make the body understand that it needs to produce an immune response. Just injecting spike protein doesn't work, because it isn't recognized as a threat. The AstraZeneca one uses an attenuated virus that the body is going to respond to anyway.
I believe the mRNA vaccines work by getting cells to express the spike protein themselves. You can't smuggle the protein into cells just by injecting it, but mRNA will get in there and do its thing.
My understanding is that the hard part is to make the body understand that it needs to produce an immune response. Just injecting spike protein doesn't work, because it isn't recognized as a threat. The AstraZeneca one uses an attenuated virus that the body is going to respond to anyway.
I believe the mRNA vaccines work by getting cells to express the spike protein themselves. You can't smuggle the protein into cells just by injecting it, but mRNA will get in there and do its thing.