Oh - I should also mention that the way the immune system responds to the bare antigen is different from the way it reacts when it's included with other potential antigens. This results in a stronger response to that particular antigen. It also means that the lethal over-response that sometimes happens will also be different, and could be more or less severe. It also means that the body's reaction to later exposure to the virus might also be more or less severe. With a hundred million people vaccinated in the US, we now have results for this "experiment". The vaccine is less harmful than the virus by a couple orders of magnitude - which is piss-poor performance and probably wouldn't have been allowed if it wasn't under a state of emergency.