Using publicly available data sources, I estimated the number of people who might have immunity in each state. The simple calculation multiplies the number of known cases in each state by 8. Then, I divided by the number of expected immune people by the state population. The numbers vary dramatically across the country, with North Dakota topping the list at 92% in contrast to the least affected state, Vermont, at a mere 7%.
A very crude estimate suggests that new cases should begin trending downward when about 60% of the population has been infected. My rough estimate showed that five states are likely to have more than 60% of their populations previously infected (North Dakota, South Dakota, Iowa, Wisconsin, and Nebraska) with three others (Utah, Rhode Island, and Wyoming) approaching 60%. In each of these states, the recent trajectory for new cases is declining. States where the seven-day rolling averages are trending upward tend to have a lower rate of previous infections: South Carolina (39%), Texas (36%), Massachusetts (34%), California (32%), West Virginia (29%), the District of Columbia (28%), and New York (17%).
We have good reason to believe that a previous infection provides immunity. The two new vaccines are estimated to be about 95% effective in preventing infections. However, those who have suffered a previous infection may enjoy greater than 99% protection. True, there have been a few cases of COVID survivors who became reinfected. But reinfection is very rare. There are only 31 documented cases among some 81 million people who have been infected. In two large vaccine trials, people taking the active vaccine were 95% less likely than those getting placebo to get COVID. But those injected with placebo were still 200 times more likely to get COVID in comparison to the rate of reinfection among COVID survivors.