Kind of seems odd, given that you can never have 0% efficacy (unless your vaccine somehow caused every vaccinated person to get the disease instead of a non-vac

Kind of seems odd, given that you can never have 0% efficacy (unless your vaccine somehow caused every vaccinated person to get the disease instead of a non-vaccinated person). It seems like you're wasting half your "spread", basically. If you measured efficacy as "1 - (vaccinated infections / unvaccinated infections)", it would be much more intuitive, and you could even get a bit of negative efficacy in the case where your vaccine did nothing.