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The coronavirus epidemic is surging and taking with it the economy, our ability to gather with friends and family, and our hopes for the future. We need to get it over with. But in the midst of the explosion in new cases, some states are experiencing declines in the number of new cases while other states are witnessing increases.
Changes in the rate of infections have been attributed to political affiliation, defiant lifestyles, and irresponsible leadership. But there may be a simpler answer. Some states may be running out of people who have not already been infected.
Experts estimate that at least two-thirds of the population need to have neutralizing antibodies in order to reach herd immunity. Two-thirds of the 325 million people in the United States is about 218 million people. You can get antibodies that neutralize the virus in one of two ways: taking an effective vaccine, which is only being starting to be administered, or having been infected with the coronavirus that causes COVID-19.
About 19 million people in the U.S. have had confirmed cases of COVID-19. But confirmed cases may be the tip of the iceberg. Although estimates vary, the Centers for Disease Control believes that about eight people have been infected for each one person with a documented case. If we multiply the 19 million known cases by 8, it is possible that about 152 million people are already immune. Yet the proportion of people who have been infected and the rate of new cases varies significantly by state.
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 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 percent in contrast to the least affected state, Vermont, at a mere 7 percent.
A very crude estimate suggests that new cases should begin trending downward when about 60 percent of the population has been infected. My rough estimate showed that five states are likely to have more than 60 percent of their populations previously infected (North Dakota, South Dakota, Iowa, Wisconsin, and Nebraska) with three others (Utah, Rhode Island, and Wyoming) approaching 60 percent.
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 percent), Texas (36 percent), Massachusetts (34 percent), California (32 percent), West Virginia (29 percent), the District of Columbia (28 percent), and New York (17 percent).
We have good reason to believe that a previous infection provides immunity. The two new vaccines are estimated to be about 95 percent effective in preventing infections. However, those who have suffered a previous infection may enjoy greater than 99 percent protection.
True, there have been a few cases of COVID-19 survivors who became reinfected. But reinfection is very rare. There are only 31 documented cases among some 81 million people who have been infected.
Some States May Already Be Close To Herd Immunity From COVID-19: