mRNA vaccines work by forcing cells to produce spike proteins which are identical to the COVID-19 spike proteins. The immune system then responds by destroying the proteins. If some people’s bodies adversely react to the spike proteins from one source then they would react the same way to the spike proteins from another source.
I do not believe it is a foregone conclusion that the human immune system see’s the spike on the virus the same as it see’s a spike produced by the cells. Remember, the immune system knows the cell produced it, also we must not forget, that with the virus the immune system see’s the spike along with the whole of the virus. With the vaccine it does not.
This is why we do years and years of testing before unleashing this type stuff on the masses. As much as humanity has learned, there is an equal share it has not learned. We must put a stop to this madness. Running a worldwide trial is not a good idea, no matter what people say.