You did a very good job of explaining how and why that works.
I’m sorry for the girl missing the prom, but here’s her life lesson. Choices have consequences. When you opt to not vaccinate, you are choosing the possibility of having to miss important events due to outbreaks.
What do they do with kids who actually had chickenpox in their early years? My kids all had it before the age of 6, and one of them had it twice since his first outbreak happened very mildly. My children never received vaccinations. What do they do with these kids? It WAS noted in their medical records from the doctor though.
Yes. They take this approach for a reason. The immunocompromised kids are going to have to stay home anyway - so if keeping them home would solve the problem without inconveniencing anybody else, that would be the best way. But it just doesn't work in practice. Especially with chickenpox, where a person becomes infectious before symptoms appear, and there's generally at least a two week gap between infection and symptoms appearing.
Unusually, it also tends to me more serious with older kids - for young kids, it's normally a minor disease. It's once you get close to adulthood, the risks increase - so kids of an age to be having a prom actually need more protection than younger kids do in this case. For most viral diseases, it's the other way around.
And the thing is, the quarantine approach doesn't just protect the immunocompromised kid - it also protects the ones who have never been immunised by choice.