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

Eight index cases complied with the inclusion criteria. In total we included 91 nominees up to the fifth generation seeded from each index case, and a total of 99 participants were enrolled into the cohort. Overall, 24.2% of participants had zero successful nominations, 32.3% had one, 26.3% had two, and 17.2% had three. Twelve (4.9%) out of the 247 nominations had already participated in the Ice Bucket Challenge and were therefore “immune” from future participation. Among those observed were a Nobel laureate, a university vice chancellor, “heart throbs”, and Muppets (both the actors and the puppets). Among the index cases, Mark Zuckerberg’s nomination chain produced the most successful number of contacts (total of 41) up to the fifth generation (figure). No serious adverse events arising from the Ice Bucket Challenge act were observed in this series, but adverse events have been reported elsewhere,16 including falls, head injuries, a temporomandibular joint dislocation, cuts, and at least one fatality. We estimated the measure of transmissibility, R0, to be 1.43 (95% confidence interval 1.23 to 1.65) and a mean serial interval of accepting the challenge of 2.1 days (median 1 day). We excluded Homer Simpson and Kermit the Frog in the regression models because of difficulty in ascertaining their personal characteristics. Participants with a higher log (base 10) net worth were more likely to spread the Ice Bucket challenge (odds ratio 1.63, 95% confidence interval 1.06 to 2.50), adjusted for age and sex. One unit change on a log (base 10) scale is equivalent to a 10-fold change in net worth. We tested the proportional odds assumption for the regression model and found no indication of violation. Age, sex, occupation, number of Facebook likes, or number of Twitter followers were not associated with transmissibility (table 2). Data were missing for 24.0% of net worth, 15.6% of Facebook likes, and 13.5% of Twitter followers. Sensitivity analysis by multiple imputation yielded similar results (not shown).