1) There was a hypothesis (reddit comment, not medical) that a degree of myocarditis might be due to people not resting for the few days after vaccination - especially young men and kids, ignoring very mild fatigue/symptoms to play sport or do physical labouring the next day - overly stressing the heart.

2) I'm surprised the dosage for moderna hasn't been reduced to 50 mcg for first and second doses (especially for young men) since it seems to provide similar protection to the full 100mcg doses: https://www.sciencedirect.com/science/article/pii/S0264410X2...

3) I'd wager we end up with more frequent, smaller doses as this goes on. Moderna at 25 mcg was shown to have some protective effect, note however pfizer didn't reach marginal gains in antibody response, with dosages instead being limited by side-effects, but still had some effect at even 1 mcg (standard dose 30 mcg) https://www.nature.com/articles/s41586-020-2814-7

4) A longer series of smaller doses might be preferable for the vaccine hesitant also - if you start getting noteworthy side-effects then you can terminate the programme.