It’s may not be justified, because the subjects are already vaccinated and protected, but there is an increase in myocarditis risk for them.

It reduces for both, but Moderna's still high at that "9+" week mark because of the larger dose. It's only reasonable to assume that it will plummet fast as more weeks pass. Notice also how the AstraZeneca vaccinated are disadvantaged yet again: they both have less protection in the first place and lose protection no matter what booster they get.

The conclusion is that the current boosters are essentially obsolete for infection control. When considering also the risk of myocarditis, vaccine mandates in colleges don't make sense.