What are your thoughts on https://doi.org/10.1161/CIRCULATIONAHA.122.061025 (AFAICT, correlation between post-vaccine myocarditis and detectable free spike, possibly due to individual differences in some adolescents causing slower(?) spike clearance)? Credible?

There's a very detailed commentary on that article you may find interesting.

https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.123.0...

I've suffered viral pericarditis before due to a cold, so I'm very aware of how the immune system can cause autoimmune inflammatory conditions (onset of my symptoms were weeks after my cold symptoms had completely cleared--the virus wasn't still around banging on my pericardium).

You don't need circulating spike to explain vaccine caused myocarditis. The vaccine produces an immune response. The initial immune response are low affinity antibodies. Those antibodies can sometimes target host cells accidentally. As somatic hypermutation happens in the months following the exposure to the antigen the high affinity antibodies that form (and which are boosted by a 2nd exposure to the antigen) are less likely to accidentally target host cells. This is presumably why I had a complete recovery after around 2 months and all I needed was an NSAID to tamp down the inflammation.

The obvious objection to that article is that correlation is not causation. It needs to be replicated, and there needs to be more than just speculation about a mechanism. There already exists a very well-known mechanism (autoimmunity) for the alternative explanation.

Well, everything in science is correlation. Causation is a construct...

I am taking your comment to mean that you think it's not a credible paper? That's the main thing I wanted to know, it seems like you believe the paper implies something very unlikely and so authors have an agenda?

You're overlooking the cases where the vaccine enters the bloodstream and itself is circulating, or gets transcribed and yields permanent spike production. Especially if combined with immune tolerance this can cause the observed effects where vaccines are directly triggering t1 diabetes (see my comments on another thread for study links showing this, including one from moderna itself).