In our debate about the notion of herd immunity related to COVID-19, we have missed something essential. When George Potter introduced the term in 1916 to describe an epidemic in Kansas, it made perfect sense. He was a veterinarian talking about cows and sheep. We are dealing with free range humans.
If 60% of a herd of cattle is immune to a disease, it doesn’t particularly matter which cattle are immune. Cows are cows. If we begin to relax controls as vaccination rates rises, we are assuming this applies to people. But look more closely.
Vaccination is not random
First the obvious. Our COVID-19 vaccination strategies have been specifically focused on protecting those at highest risk of dying first. It made perfect sense to begin by immunizing the elderly or those with underlying conditions. It also made sense to preferably protect health care workers and others at high risk of infection. But then, as rates of death and disease dropped, we acted as if we were vaccinating a herd of cattle. We assumed that any subgroup we selected had the same level of immunity. We took the one segment of the population that we had specifically not vaccinated, the group least likely to have been previously infected because they rarely left the house, and sent them back to school.
The desire to return kids to the classroom is understandable, but the premature opening of schools poses a risk. Evidence suggests it has been a driving force in the third wave of disease. Anyone who has ever had young children knows that they are a key vector in the spread of disease. The rush to reopen schools has meant that we have taken the group with the lowest level of immunity and the lowest likelihood of complying with COVID safety guidelines and clustered large numbers of them indoors for extended periods, the conditions most likely to encourage spread of disease.
Since schools reopened, we have seen rising rates of COVID-19 among children. On top of that, we have seen dramatic rises in the proportion of cases coming from the age group most likely to be the parents of those students, young and middle-aged adults. In light of the obvious risks described above, we do not get to be surprised.
And this is not the only way the herd immunity metaphor falls apart. Among any eligible demographic category, vaccination is not random. If you don’t have a car, you can’t drive to the mass vaccination site. If you don’t have an internet connected computer, you might not even know where it is. Perhaps most importantly, it now appears that a large segment of the population will refuse to seek vaccination.
To make matters worse, vaccine hesitancy occurs in a particular sub-population. The extremist “experts” and reckless, uninformed media pundits spouting anti-vaccine nonsense tend to be the same ones discouraging the use of masks and dismissing the risks of indoor gatherings. In this case, their followers do act like a herd. Of sheep. Those least likely to seek vaccination are also those most likely to expose themselves to COVID-19 safety guidelines. And to frequent the same events. For examples of this, we need look no further than the Sturgis Rally and the Rose Garden super-spreader event. This collective bad behavior means we could inoculate 90% of the US population and the 33 million left could sustain the disease indefinitely.
The herd immunity debate
That same polarization plays out in the debate about herd immunity. The public forum has boiled it down to two simple issues. What percent of people need to be immune and how do we get there. Reality is far more complicated. Nuance is essential. Simplification can have deadly consequences.
It would be nice for this purpose if people acted like George Potter’s cattle, but they don’t. To the extent the concept of herd immunity remains useful, we ned to rethink it. We must recognize the complex interplay between behavior, immunity, and risk in different subpopulations and adjust our strategy accordingly.
When George Potter coined the term, he advised that achieving would require “retaining the immune cows, raising the calves, and avoiding the introduction of foreign cattle”. By “foreign”, he was not referring to Mexican milk cows bypassing the border wall. He was talking about any group of cattle that might introduce the infection. You can know the risk in your own herd, but not in others, particularly if the follow different safety rules. Herds that engage in high risk behavior and a have lower levels of immunity (vaccination) pose a particular risk.
In sum, not all herds are created equal. Manage your herd well and mix herds with caution.