Quantifying social distancing arising from pandemic influenza

TLDR

Local epidemic curves during the 1918–1919 influenza pandemic were often characterized by multiple epidemic waves. The study aimed to determine whether these waves were driven by people avoiding potentially infectious contacts—a behavior termed “social distancing.” The authors estimated the effective reproduction number and inferred the maximum degree of social distancing that occurred during the multiple‑wave epidemic in Sydney, Australia. Analysis shows that a 38 % reduction in infectious contacts—sufficient to explain the multiple waves—was accompanied by an R₀ of about 1.8, a 36.6 % clinical attack rate versus an expected 59 %, and an estimated 260 lives saved per 100 000, indicating that social distancing could substantially mitigate future influenza pandemics.

Abstract

Local epidemic curves during the 1918–1919 influenza pandemic were often characterized by multiple epidemic waves. Identifying the underlying cause(s) of such waves may help manage future pandemics. We investigate the hypothesis that these waves were caused by people avoiding potentially infectious contacts—a behaviour termed ‘social distancing’. We estimate the effective disease reproduction number and from it infer the maximum degree of social distancing that occurred during the course of the multiple-wave epidemic in Sydney, Australia. We estimate that, on average across the city, people reduced their infectious contact rate by as much as 38%, and that this was sufficient to explain the multiple waves of this epidemic. The basic reproduction number, R 0 , was estimated to be in the range of 1.6–2.0 with a preferred estimate of 1.8, in line with other recent estimates for the 1918–1919 influenza pandemic. The data are also consistent with a high proportion (more than 90%) of the population being initially susceptible to clinical infection, and the proportion of infections that were asymptomatic (if this occurs) being no higher than approximately 9%. The observed clinical attack rate of 36.6% was substantially lower than the 59% expected based on the estimated value of R 0 , implying that approximately 22% of the population were spared from clinical infection. This reduction in the clinical attack rate translates to an estimated 260 per 100 000 lives having been saved, and suggests that social distancing interventions could play a major role in mitigating the public health impact of future influenza pandemics.