You're right. Can't just post them to folks and expect 100% uptake. It might widen the possibilities of more people getting more vaccines, though. In my books, this can only be a good thing.
I agree! Auto injectors aren't cheap compared to ye olde trusty ampule and syringe, and this might push the costs towards the higher end again. I can see a kids-and-the-latex-allergic edge case scenario.
Maybe one way of looking at it might be : this would be safe enough you could trust people to self-administer, and you could therefore take the professional with the needle out of the equation.
90 seconds of one person's time has got to be better than the quick jab by two people, no?
You say that, but there's such things as raves for children. If you have kids who like to dance, they are epic fun for them.