Edit: Read Post #982 first.
This post presents a calculation to match the herd immunity discussion of the just-prior post. Read Post #978 first, then this one.
Here, I back-solve for the level of immunity in the population that should bring the effective COVID-19 viral reproduction factor below 1.0 (i.e., end the third wave of the pandemic), as long as we maintain masking, distancing, and other behaviors limiting viral spread.
This is a simple calculation, based on one point in the progress of the pandemic in North Dakota. That point being the two weeks when North Dakota saw its sharpest increase in cases.
So there’s not a lot of accuracy here. And it’s not an estimate, in the sense of being a statistic calculated from pooling a lot of data. It’s really just a round-numbers (but data-based) illustration. It shows that the two different herd immunity concepts defined in the prior post lead to two very different levels of required population immunity. And that we may already be hitting the lower level in some states.
Bottom line: 40%. Once something like 40% of the population has been infected, that ought to be enough to set the third wave of COVID on a downward trajectory. As long as we maintain masking, distancing, limits on social gatherings, and other such controls. But we’d still need the classic “70% herd immunity” to return to normalcy, meaning, life without those controls.
The upshot is that the uniformly downward trajectory seen in the U.S. Midwest probably isn’t a fluke, or luck. It’s probably just a matter of arithmetic.
The clear policy implication is that there is a more efficient way to use the COVID vaccines, if the goal is to bring the U.S. third wave of COVID to a close. You should concentrate vaccination in those states that have had the fewest infections so far. You shouldn’t aim for an equal share of the population vaccinated in each state, as we are now. You should aim for an equal share immune in each state, either via vaccination or via prior infection. That means shifting vaccine from states that have already had widespread COVID infection, to states where a higher fraction of the population still lacks immunity to the virus.
Continue reading Post #979: The two distinct levels of herd immunity, Part II