Interesting! That does look promising. South Dakota has 762k people and 125k positive tests. The CDC assumes that 1 in 4.2 infections were reported, so that would be 525k people infected total. 52.8% of people got vaccinated. Assuming there's no correlation between getting vaccinated and getting infected (which I'd assume isn't 100% correct), you can assume that of the 295k uninfected, 156k got vaccinated. So that'd be a total of 525k infected+156k vaccinated and not infected = 681k = 89.4% immunity.