Spike proteins are protein protrusions expressed on the virus capsule, glycosylated S proteins to be precise. Made up of two subunits, S1 mediates cell recognition and S2 mediates cell membrane fusion. These subunits cause what we call a conformational change to your cell membrane, something and this causes irreversible membrane fusion. On the virus itself, the spike proteins are coated with polysaccharide molecules to essentially camouflage itself from our immune system and this is where the damage is done, because by the time the immune system recognises there is a problem, covid has already transformed your cells into a covid making machine, this is where you get sick because your cells are not only under stress (because covid is literally chopping your proteins up) but your immune system declares war and immune mechanisms of action cause significant damage.

In regard to the mRNA vaccine, it holds information for your cells to make this protein but not just any old cell, specifically your immune cells. Not every cell is capable of expressing different proteins presented to it, it is your immune cells that do this and they are primed for the job, other cells just don’t have the means to do that, if they did then our bodies would be a gigantic mess. The key differences in a covid infection and an mRNA vaccine is 1) the subunits are modified so an irreversible conformational change will not happen and your cells can’t be transformed into a viral machine and 2) it is instantly recognised by your immune system because it is not camouflaged by polysaccharide molecules, leaving you with the correct antibodies to be prepared for exposure but none of the nasty symptoms.

I also want to clarify that covid doesn’t get your cells to express its proteins, it gets inside your DNA and makes your cellular machinery make more of it. An mRNA vaccine does not do this as it does not have any mechanism to do that.

It is wildly complicated.