There is a very small vein behind the delta muscle, direct at the bone - extremely hard to hit.
Myocarditis and platelet aggregation become a bigger problem with the second shot. This is when the body is already trained to produce a huge amount of antibodies as soon as the antigen is detected. For the blood clotting, they could already show that antibodies are the culprits, one type of antibodies binds to a region at the platelets that triggers their aggregation ("vaccine-induced immune thrombotic thrombocytopaenia"): https://www.nature.com/articles/s41586-021-03744-4
They haven't found the cause of the myocarditis yet, but I bet it is caused by antibodies too.
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