Here’s a summary of this paper…
The mRNA COVID vaccines (but not the adenovirus ones) contain contamination from double-stranded RNA due to the manufacturing process. Double-stranded RNA is known to be inflammatory as it sets off the TLR3 receptor.
Note: Vaccines arguably have to be inflammatory to trigger the immune system into developing immunity. We don’t know if some ways of triggering the immune system are safer than others. Various components of the vaccine are known to be inflammatory- e.g. the single stranded RNA which was modified to be less inflammatory, the lipid nanoparticles, etc.
Hot take: This paper doesn’t really explain why dsRNA leads to myocarditis. It’s just a wild hypothesis.