title: Transmissibility of the Ice Bucket Challenge among globally influential celebrities: retrospective cohort study.
abstract: OBJECTIVE
  To estimate the transmissibility of the Ice Bucket Challenge among globally influential celebrities and to identify associated risk factors.
  
  
  METHODS
  Retrospective cohort study.
  
  
  METHODS
  Social media (YouTube, Facebook, Twitter, Instagram).
  
  
  METHODS
  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.
  
  
  METHODS
  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.
authors.affiliation: School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China nimy@hku.hk.
authors.forename: Michael Y
authors.initials: MY
authors.lastname: Ni
authors.affiliation: School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: Brandford H Y
authors.initials: BH
authors.lastname: Chan
authors.affiliation: School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: Gabriel M
authors.initials: GM
authors.lastname: Leung
authors.affiliation: School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: Eric H Y
authors.initials: EH
authors.lastname: Lau
authors.affiliation: School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong Special Administrative Region, China.
authors.forename: Herbert
authors.initials: H
authors.lastname: Pang
mesh_terms: D000690:Amyotrophic Lateral Sclerosis / Q000191:economics; D035843:Biomedical Research / Q000191:economics; D005197:Famous Persons; D005653:Fund Raising / Q000379:methods; D006801:Humans; D007053:Ice; D033181:Information Dissemination / Q000379:methods*; D011639:Public Opinion; D012189:Retrospective Studies; D012307:Risk Factors; D061108:Social Media
chemical_list: D007053:Ice