1. This would require testing tons of people for antibody response all the time. That seems hard. We do at least have antibody tests that will test for infectio

1. This would require testing tons of people for antibody response all the time. That seems hard. We do at least have antibody tests that will test for infection vs vaccination. 2. If you are going to go this far, you'd also want the "vaccinated but had no effect" group. The vaccine will not generate an immune response (or an effective one) in everyone, that's just how it works. So the vaccinated group (and "breakthrough cases") include some percent of people who were vaccinated, but it didn't really do anything to them. You'd want to know this number, because you want to know things like "how many breakthrough cases are people where the vaccine had no real effect vs people where the vaccine generated an effective antibody response, but not against this variant" Especially because the "breakthrough" rate is small.