What Is Herd Immunity?

Herd immunity and epidemic control

Before the Covid-19 pandemic struck, few non-epidemiologists had come across the concept of "herd immunity." That changed when some governments made it the basis of their initial pandemic control strategies. The idea was that when enough people become infected and develop immunity, community transmission of the SARS-CoV-2 virus would end. Immunologists, appalled, immediately pointed out that no one knew what percentage of the population would have to be immune. It could be anywhere from 60 percent to more than 90 percent. Long before we’d begun to approach anything like that, the mortality rate among the elderly and immune-compromised would have reached utterly unacceptable levels. Policymakers changed course and the concept disappeared from news headlines.

The concept of herd immunity

The term herd immunity is somewhat unfortunate. A herd usually refers to animals such as cows, goats, and sheep, which are kept by farmers and then quite possibly killed and eaten. The term seems to have been introduced, a century ago, by American veterinarians concerned about epidemics of spontaneous miscarriage in cattle and sheep. Only in the 1970s did it find its way into epidemiological studies of human infectious diseases. Thinking of oneself as part of that kind of a herd is somehow demeaning. Think herd mentality! But does the term actually matter?

Whether it does or not, the questions raised about the meaningfulness of a single target number certainly do. A number such as 60 percent or 90 percent depends on the assumption that populations (herds) are homogeneous: that each of us mixes randomly with all other members of our herd. I don’t know how far this is true of cows or sheep but it is quite obviously untrue of humans. In modern societies, people don’t interact randomly with everyone around them. We have our social networks, our neighborhoods, our communities. The past two years have demonstrated with tragic clarity that these differ in the speed with which the SARS-CoV-2 virus spreads through them. The idea of a single safety threshold, a percentage, suggests a clear target, however hard to reach. But because our interactions with each other are clustered and heterogeneous, epidemiologists have acknowledged its limitations. Allowing for this, deriving more complex real-world metrics, has become a topic of fascination for modelers and game theorists.

‘Who cares?’

Modeling the spread of the virus in a community is one thing. Living in the community is quite another. Which of us cares about the vaccination status of people around us, or the extent of immunity in our communities? And if we do care, how do we express it?

In an earlier post, I described questioning a few friends, demographically similar but living in different countries. Had their behaviour been affected by the vaccination status of people around them? One or two said that yes, their social and even family relationships had been significantly disrupted. In a German study, a group of behavioral scientists explored a related question more rigorously. They started from the idea that because it helps protect the vulnerable, we can think of vaccination as involving a moral obligation to our communities: a social contract. In four quasi-experimental studies, carried out online, they looked at whether individuals who are vaccinated (and therefore comply with the social contract) behave more generously toward other people who have been vaccinated compared with those who have not (and so violate the social contract). And indeed, they found that vaccinated participants showed lower levels of generosity toward non-vaccinated others, suggesting some basis of a social contract.

Prosocial vaccination

What about actual vaccination decisions? Do people know about herd immunity? Largely speaking, no, they don't. At least prior to the pandemic, most people were unfamiliar with the concept. Do they care? Are vaccination decisions influenced by a sense of obligation, a social contract with the community? Is vaccination seen as a collective task involving shared responsibility for vulnerable neighbors?

On the whole, no. In one study, for example, participants reported that their personal decisions were unaffected by whether people around them were vaccinated or not. Even among those who said they were influenced by their social network, few referred to herd immunity specifically. Some people recognized that childhood immunization had indirect benefits to the community. Nevertheless, they thought about immunizing their own children in terms of what they saw as their child's benefit.

Making vaccination decisions in the light of the collective interest, now known as "prosocial vaccination," seems to be rare. Why should this be? Based on a study of parents living in the San Diego area, anthropologist Elisa Sobo offers an intriguing answer, which she relates to Robert Putnam’s classic study of declining social engagement. Sobo’s study participants explained that they couldn’t rely on other parents to hold up their end of the social contract. Parents who vaccinate, in this context, felt they couldn’t count on others to work with them in trying to achieve a collective, community-level goal. “My findings,” Sobo writes, “suggest that perceived ties to the broader collective, public health’s herd, are weak among today’s selective vaccinators, at least in the US context”.

These American parents are not alone. Whether or not because the indirect effects of vaccination are not known, the well-being of the ’herd’ appears as a minor consideration in most studies of vaccination decision-making. Studies suggest that in most societies vaccination decisions are rarely prosocial, and vaccination is not perceived as a collective responsibility. (There’s a bit of evidence that people in Asian countries such as South Korea and Vietnam and in countries with a strong social welfare tradition such as Sweden, may see this differently.)

Most studies were carried out pre-Covid-19. But have we become more prosocial? It’s now being suggested that emphasizing vaccination’s benefits to the wider community could help overcome vaccine hesitancy. But how far this will resonate? Its likely impact will surely depend on how, if at all, people in any given community already relate to each other.