If I recall correctly from the early days of the virus, myocarditis was also proposed as a long term side effect of the virus itself.
Why would the vaccine and the virus has so many of the same side effects?
I don’t know for sure; but if I had to guess I would say, “Spike Proteins”....which the vax produces and which are implicated in many covid symptoms.
Most likely, similar inflammation is caused by immune reactions to proteins generated by both the virus and by the vaccine.
Because the spike protein itself is a pathogen. The mRNA "shot" is make spike protein factories all over the body of the recipient. Unlike the natural infection, this mRNA injected spike protein lacks the other control mechanisms bundled with the natural SARS-CoV-2 viral infection. Part of the missing controls removes the inhibition of keeping the spike protein from expressing on the surface of the host cell. There is a long list of errors in the thought process in generating the mRNA "vax"
It's not a vaccine, since it doesn't contain the whole virus; but the jab and the real virus have the spike protein in common.
When the spike protein interacts with the ACE2 receptor on human cells, all kinds of inflammatory signaling molecules are produced / released by the cell whose receptor it is.
One of the signaling molecules is described below; I'm not saying this particular one is related to heart inflammation, though.