Herd immunity: recent uses in vaccine assessment

Abstract

Human communities defend themselves against specific infectious agents in a way that extends beyond the simple sum of the immune status of its individuals. By analogy with individual immunity to specific agents, the community level of immunity may vary from complete susceptibility to full protection. Herd immunity has been used to name this community property, which is the result of evolution through natural selection, leading to relationships between two species, typical of prey–predator systems. Varying uses of the term herd immunity led to the use of other expressions, such as herd protection, herd effect and community immunity. Knowledge derived from observational studies and models on herd immunity has supported decisions on the choice of vaccines and vaccination strategies for the benefit of populations. This knowledge is most likely to be extended in the future, with far-reaching effects.

Keywords::

• community immunity
• herd effect
• herd immunity
• herd immunity threshold
• herd protection
• vaccination

Acknowledgements

I thank Rui Fontes and Zaida Yadon, for valuable comments on a previous version of this manuscript.

I am grateful to Prof. Joaquim da Costa Maia for his support and inspiration throughout my career. He died, aged 85, while this article was being written. He was taught by Reed and Frost at Johns Hopkins University, and his work, published in 1952, was quoted in two of the review papers mentioned in this article. Reading his considerations on the importance of epidemic theories was a great lesson for me.

Financial & competing interests disclosure

The author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.