Granted, But 74% ?!?! Apparently in a place with a 75% vaccination rate? Why isn't the infection rate highly skewed towards the un-vaxxed??
Let's show an example: If you have 100 people in a room and 10 are unvaccinated and 90 are vaccinated - let's say you have a 50% chance of getting infected from a single person (for sake of argument) without vaccination, and with vaccination, 80% less than that. That would mean in that crowd, about 5 unvaccinated people would get infected, and 9 vaccinated people, for a total of 14 people, mostly vaccinated.
But if you reversed that (90 unvaccinated and 10 vaccinated), 45 unvaccinated people would get infected and 1 vaccinated person would get infected - for a total of 46 people infected, overwhelmingly unvaccinated.
So the higher the ratio of vaccinated to unvaccinated in any situation, you can have more vaccinated being infected.
Now, with low vaccination rates, those 45 people now can spread it to other unvaccinated and vaccinated exponentially. In a high vaccinated scenario, the vaccinated will still spread it, but again at a much lower rate - as in every spreading opportunity, the risk of spreading to another vaccinated person is 80% less of that of spreading it to an unvaccinated person. So if everyone is vaccinated, cases go down exponentially, not to mention the risk of hospitalization and death, which the vaccines have a much higher efficacy against.
Bottom line - the more people vaccinated, the more vaccinated people will catch mild illness, but the more it will spread.
With Delta's R0 factor it's likely we need about 80-85% vaccinated for herd immunity - and that's of the whole population, not just those currently eligible.