Transmissibility of the Ice Bucket Challenge among globally influential celebrities: retrospective cohort studyBMJ 2014; 349 doi: https://doi.org/10.1136/bmj.g7185 (Published 16 December 2014) Cite this as: BMJ 2014;349:g7185
- Michael Y Ni, clinical assistant professor,
- Brandford H Y Chan, research officer,
- Gabriel M Leung, chair professor,
- Eric H Y Lau, assistant professor,
- Herbert Pang, assistant professor
- 1School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China
- Correspondence to: M Y Ni
- Accepted 17 November 2014
Objectives To estimate the transmissibility of the Ice Bucket Challenge among globally influential celebrities and to identify associated risk factors.
Design Retrospective cohort study.
Setting Social media (YouTube, Facebook, Twitter, Instagram).
Participants David Beckham, Cristiano Ronaldo, Benedict Cumberbatch, Stephen Hawking, Mark Zuckerberg, Oprah Winfrey, Homer Simpson, and Kermit the Frog were defined as index cases. We included contacts up to the fifth generation seeded from each index case and enrolled a total of 99 participants into the cohort.
Main outcome measures Basic reproduction number R0, serial interval of accepting the challenge, and odds ratios of associated risk factors based on fully observed nomination chains; R0 is a measure of transmissibility and is defined as the number of secondary cases generated by a single index in a fully susceptible population. Serial interval is the duration between onset of a primary case and onset of its secondary cases.
Results Based on the empirical data and assuming a branching process we estimated a mean R0 of 1.43 (95% confidence interval 1.23 to 1.65) and a mean serial interval for accepting the challenge of 2.1 days (median 1 day). Higher log (base 10) net worth of the participants was positively associated with transmission (odds ratio 1.63, 95% confidence interval 1.06 to 2.50), adjusting for age and sex.
Conclusions The Ice Bucket Challenge was moderately transmissible among a group of globally influential celebrities, in the range of the pandemic A/H1N1 2009 influenza. The challenge was more likely to be spread by richer celebrities, perhaps in part reflecting greater social influence.
We thank Y Yuan, CKL Yiu, and T Li for their contributions to the data collection (which we hope provided some entertainment) and the illustration of the Christmas trees.
Contributors: MYN conceived the study, wrote the initial protocol, collected data, and wrote the first draft of the manuscript. BHYC developed the protocol and collected data. EHYL, HP, MYN, and GML analysed and interpreted the data. All authors critically revised the final manuscript, had access to the full dataset, take responsibility for the integrity of the data and the accuracy of the dataset, and gave final approval for the submission of this version for consideration of publication. EHYL and HP contributed equally to the work. MYN and BHYC are guarantors for the study.
Funding: This study received no external funding.
Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare: no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.
Ethical approval: Not required.
Data sharing: The dataset is available at the Dryad Digital Repository (provisional doi:10.5061/dryad.n4sc4).
Transparency: The lead author (MYN) affirms that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.
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