The R₀ journey: from 1950s malaria to COVID-19

News coverage of the COVID‑19 epidemic makes frequent reference to the reproduction number, R₀, the average number of new cases of a disease that arise from a single case. As well as recognizing its simple mathematical power and the challenges its use poses (see C. Uzoigwe Nature 582, 341; 2020), it is important to understand how it originated.

The R₀ concept has been attributed to the late Robert May (1936–2020). Although May championed R₀ and contributed to its application (see R. M. Anderson and R. M. May (eds) Population Biology of Infectious Diseases; Springer, 1982), it was first developed more than 60 years ago by the epidemiologist George Macdonald, then director of the Ross Institute of the London School of Hygiene and Tropical Medicine. His aim was to understand quantitatively the transmission of malaria, a mosquito-borne disease (G. Macdonald The Epidemiology and Control of Malaria; Oxford Univ. Press, 1957). He derived R₀, originally designated Z₀, from a reproduction ratio established by the demographer Alfred J. Lotka (see D. L. Smith et al. PLoS Pathog. 8, e1002588; 2012) .

The number became known as R₀ in the 1970s, and has since been widely applied in disease epidemiology.