Herd Immunity is not an Individual Decision

By Foss Tighe

In an number of prior posts I have discussed how Herd Immunity could be a valid public health strategy for dealing with COVID-19. In a recent post I outline how limited herd immunity combined with other outbreak mitigation strategies could work together to slow and reverse the pandemic. I also explore the possible value of a herd immunity strategy in my post on relying too heavily on a vaccine. But as I discuss in my post on our limited “budget” for opening up society again, these decisions are ones that must be made on a society/community wide basis to be meaningful. It should be pretty obvious just from the term “public health emergency”, that the solutions to the problem are “public” in nature.

If you turn out to have Diabetes, your strategy for dealing with the disease is pretty much a personal one. For the most part, the rest of society does not really care if you decide to try exercise and weight loss or if you want to take medications. Certainly there is some indirect societal interest, as your decisions will likely have health care cost implications, and some choices might cost the health care system more than others, and therefor might influence health care costs at some point down the road. But this is all very indirect. But with COVID-19, there is an interdependence between all members of the community. Actions by one individual can influence outcomes for others. One result is that most rational strategies for dealing with COVID-19, including herd immunity, need to be executed on large scale populations.

Herd immunity is not an individual strategy, nor is it a valid strategy for small groups of people, such as a family, school, apartment complex, or even an entire town. It is after all a herd strategy. For most herd animals, the herd represents the community in which they spend the vast majority of their time. Kids in the herd rarely hang out with kids from other herds, down at the river.

Just to review, the basic idea behind herd immunity is that for an infectious viral disease like COVID-19, there is a point at which, once enough members of the herd have been infected, recovered and acquired immunity, that the number of infections will start to decline and will eventually go to zero. This will happen long before 100% of the herd have been infected because of the dynamics of viral infections. See the discussion of R-naught in my post on the other side of the curve. Many public health experts have suggested the point at which infections will decline and eventually disappear in our human herd with COVID-19 would be somewhere between 60% and 80%. In more recent research some people have suggested herd immunity could take effect with even lower infection rates. But in any case once this happens you still have a large percentage of the population who have not had the disease, but they are now more or less protected by that fact that so many members of the herd are now immune that any new outbreak will quickly die out for lack of susceptible members to support an ongoing outbreak. That is the glory of herd immunity.

The problem with herd immunity in smaller populations is that if members of the smaller community routinely interact with people outside of their “herd” the protection provided by their herd does not extend to those outside interactions. Consider a school with 100 students and staff. If the school were completely isolated from other social interactions, its possible that once 60 people in the school had recovered from COVID-19 the other 40% would be relatively protected. But if members of that 40% of the students or staff had any significant interactions with people outside of the school community, such as going home to their families at the end of each day, they would still be at risk for infection. Perhaps a live-in school on an isolated farm in rural Montana could experience the advantage of herd immunity as long as none of the students or staff ever went ever visited their families.

To actively promote that people behave in a manner designed to achieve herd immunity in a non-isolated community, while the larger community does not also pursue herd immunity brings no benefit to the smaller community. In fact it places the entire community at a higher risk of infection and increases the risk that community members will experience the adverse outcomes that are possible from that infection. All the while not providing any of the long term benefits of herd immunity. It is also irresponsible, because it will weaken and undermine that public health strategy of the larger community.

COVID-19 is a public health issue and requires a public response. This is a frustrating situation in a nation that cherishes individual choice over collective action. But we need to get over it and realize that our entire country needs to pursue a unified strategy toward COVID-19. We need healthy debate, but we need to keep the discourse civil, and whether we agree or disagree with the strategies currently being pursued by our elected officials and our public health officials, we should all feel bound to abide by those choices until elections or changes in our understanding of the science cause the public health experts to choose new strategies.

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