Transmissibility of the Ice Bucket Challenge among globally influential celebrities: retrospective cohort study

Ni, Michael Y; Chan, Brandford H Y; Leung, Gabriel M; Lau, Eric H Y; Pang, Herbert

The Ice Bucket Challenge, a campaign to raise awareness and support for people with amyotrophic lateral sclerosis,1 is widely recognised to have gone socially viral. Although there are variations to the trial, participants typically complete the challenge (they are doused in ice water or make a donation) and nominate three others to undertake it.2 As of 1 September 2014, more than 17 million videos related to the Ice Bucket Challenge were shared on Facebook alone, and these were viewed more than 10 billion times by more than 440 million people.3 Socially viral phenomena include videos that are made popular by sharing on the internet, and a video is said to have gone viral if it spreads rapidly as a result of frequent sharing.4 5 However, the infectious disease modelling framework has seldom been used to quantify the transmissibility of such socially viral phenomena.6 The most commonly used metric of transmissibility is the basic reproduction number (R0), defined as the number of secondary cases generated by a single index in a fully susceptible population.7 The value of R0 is a major determinant of the size of an epidemic, and an infection can only be self sustaining if R0 is greater than 1. The R0 also provides a measure of the effort required to control the epidemic.7 8 We estimated the transmissibility of the Ice Bucket Challenge among globally influential celebrities and identified the associated risk factors.