Great question, Ruth. I wonder if Laura Downey or Isabel Osborne have some thoughts (they may not be on this board).
I think the stickers are a good system for in-person events potentially, but I wonder if people will feel self-conscious/in the public eye when they do that. It could go either way - people may feel pressured to say yes if an educator is standing there watching, or may not want to participate if their response is too "public."
I wonder if something like a QR code (and link) to enable people to do this confidentially would be effective? Maybe an incentive could be coupled to a text (or other) reminder to go get their shot, maybe with link to where to schedule or other resources also? But I don't know whether that would capture people who a) already got a shot or b) don't want to get one/aren't fully convinced (though maybe the latter could get a gentle followup with "do you have more questions"?) ....
But these systems take some tech knowhow to create and time to manage.