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One common definition of herd immunity—a concept pervasive during the COVID-19 pandemic—is the protection that non-immune people can get from an infectious disease when enough other people in their population are immune because of a previous infection or vaccination. Harvard Public Health spoke with David Robertson, a postdoctoral history researcher at the University of Oxford in Oxford, England about a recent study of the history of the concept.

Why study this topic?

During the first year of the COVID-19 pandemic, herd immunity became a popular and politicized term. Yet throughout history, there have been many definitions of herd immunity, which has led to confusion about how the term should be applied and which disease control strategies to use.

What did you find?

We found that the term “herd immunity” has been in use for over a century, often used in situations where no vaccines were available—showing that herd immunity has never been dependent on vaccines (although of course, they are important). We also pointed out that discussing populations (instead of individuals) as “immune” during the COVID-19 pandemic likely led to ambiguity and confusion.

What would you like to see happen based on the results?

I hope that if we are ever faced with a similar pathogen, we can think differently about who is most at risk and how to leverage herd immunity to protect them, whether or not vaccines are available. I also hope there is more awareness that herd immunity isn’t a “let it rip” strategy but rather captures our social nature and uses it to protect the most vulnerable in society.

—Leah Rosenbaum

(Study in Journal of Evaluation in Clinical Practice, April 2024)

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