LAUSR

In this first pandemic year of an infectious disease (COVID-19), it seems particularly apt to recall that the foundations for controlling and eventually eradicating another devastating infectious disease – smallpox – began during the 18th century. I can draw on a vast secondary literature to briefly recount this history. It is relevant, for it will deploy this important early example of probabilistic thinking in the history of evaluation of a medical measure, and how this thinking was related to quantification. Between 1715 and 1721, smallpox had killed onefourteenth of the population of London. Variolation – the inoculation of smallpox lymph into the skin of healthy people as a preventive measure against smallpox (Miller, 1957) – was an oriental and North African practice (Boylston, 2012). In Europe, it was first used in Britain in the 1720s. Thomas Nettleton (b.1683; Boylston, 2010), a physician in Halifax and one of the earliest to carry out mass smallpox inoculation, calculated the outcomes in terms of death rates: the death rate of naturally acquired smallpoxwas ‘near one fifth’ (636 out of 3405) whereas it was none out of 61 inoculated persons (Nettleton, 1722). This was an unconsciously expressed probabilistic statement. James Jurin (b.1684), Secretary of the (London) Royal Society, and a Cambridge MA and MD with a good mathematical education, was motivated by Nettleton’s observations to solicit reports of personal and professional experiences with variolation from readers of the Philosophical Transactions of the Royal Society. From 1721, he received over 60 replies from physicians and surgeons and summarised them in a series of annual pamphlets (Bird, 2018; Jurin, 1724). Jurin’s analysis concluded that the chance of death from variolation was roughly 1 in 50, while the chance of death from naturally contracted smallpox was 1 in 7 or8 (Bird, 2017, 2018).Thiswas a further exampleof an unconsciously expressed informal probabilistic statement, implying a mode of probabilistic thinking. After Jurin’s death, the revolutionary technique of systematic collection and computation was continued in London by a Swiss, Johann Caspar Scheuchzer (b.1702), who presented his data in tabular form (Scheuchzer, 1729). Similar tabular data were also produced by an American, Zabdiel Boylston (b.1679; Boylston, 2008a; Boylston and Williams, 2008), who, in his 40s, had travelled from Boston to present them to the Royal Society in 1725! Such actuarial data were published in the Philosophical Transactions and widely circulated throughout Europe, but they did not end controversies over the propriety and efficacy of smallpox inoculation. Dependence on data collected was doubted: Could one trust in numbers? More data were needed. But there was also opposition of other kinds: concerns about contagiousness of inoculated persons were raised; and religious fatalists saw inoculation as a blasphemous attempt to escape God-sent providence. Eventually, however, inoculation became widely adopted during the 18th century. By the end of the century, calculation had been used to evaluate the results of controlled clinical trials (Boylston, 2008b), and mathematics had even been deployed to guide contact tracing and prevent spread of the disease (Haygarth, 1784, 1793). Vaccination (inoculation with cowpox) had been identified as an even safer way of protecting people from the disease (Boylston, 2012). Using these approaches developed in the 18th century, smallpox was eventually eradicated 200 years later. These 18th-century numerical evaluations of healthcare interventions led to a fundamental debate on the applicability of a formal calculus of probabilities in decisions related to medical treatments. Journal of the Royal Society of Medicine; 2020, Vol. 113(7) 274–277