The more vaccinated a population, the more we’ll hear of the vaccinated getting infected. For example, say there’s a community that’s 100% vaccinated. If there’s transmission, we know breakthrough cases will happen. So, by definition, 100% of outbreak cases will be among the vaccinated. It will just be 100% out of a smaller number.
Cue Israel. They are one of the global leaders in vaccinations; 85% of Israeli adults are vaccinated. So, say we have the following scenario:
With an infection rate among the vaccinated of 2% and infection rate of 13% among the unvaccinated, this would give us an efficacy rate of 85%.