The great balancing debate: a history of observing and cultivating herd immunity, 1920-2020.

In this article, I examine the history of the concept of herd immunity, beginning with British epidemiologists in the 1920s and ending with the controversy surrounding it during the COVID-19 pandemic. I argue that competing historical and contemporary understandings of herd immunity reveal an underlying tension between observing the effects of infection-acquired herd immunity on the population dynamics of infectious diseases and actively cultivating it through immunisation. Originally offering an explanatory mechanism for the rise and fall of epidemics, the concept soon became entangled with strategies of disease control and technologies for producing immunity, particularly as mass vaccination became more common in the postwar era. This tension between observing herd immunity and cultivating it has produced diverse interpretations ranging from the temporary abatement of an outbreak due to the accumulation of infection-acquired immunity to the principle undergirding disease elimination through mass vaccination. I close by suggesting that the scientific debates and uncertainties regarding the relevance of herd immunity to public health strategies during the COVID-19 pandemic reflect this long-running tension between observing and cultivating immunity in populations.