MEDIA STUDIES

Transmissibility of the among globally influential celebrities: retrospective cohort study

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

School of Public Health, Li Ka Shing OBJECTIVES To estimate the transmissibility of the Ice The most commonly used metric of transmissibility is Faculty of Medicine, The University Bucket Challenge among globally influential celebrities the basic reproduction number (R0), defined as the num- of Hong Kong, Hong Kong Special and to identify associated risk factors. ber of secondary cases generated by a single index in a Administrative Region, 7 Correspondence to: M Y Ni DESIGN Retrospective cohort study. fully susceptible population. The value of R0 is a major [email protected] determinant of the size of an epidemic, and an infection SETTING (YouTube, , , Cite this as: BMJ 2014;349:g7185 can only be self sustaining if R is greater than 1. The R Instagram). 0 0 doi: 10.1136/bmj.g7185 also provides a measure of the effort required to control PARTICIPANTS David Beckham, Cristiano Ronaldo, the epidemic.7 8 We estimated the transmissibility of the , , Mark Ice Bucket Challenge among globally influential celebrities Zuckerberg, Oprah Winfrey, Homer Simpson, and Kermit and identified the associated risk factors. the Frog were defined as index cases. We included contacts up to the fifth generation seeded from each index case and Methods enrolled a total of 99 participants into the cohort. Participants MAIN OUTCOME MEASURES Basic reproduction number We considered globally influential celebrities who had R0, serial interval of accepting the challenge, and odds ratios undertaken the Ice Bucket Challenge to be eligible for inclu- of risk factors based on fully observed nomination chains. sion. Global influence was defined by the criteria: listed in RESULTS Based on the empirical data and assuming a TIME 100: The Most Influential People in the World9 or TIME: 10 branching process we estimated a mean R0 of 1.43 (95% Great People of the 20th Century, or having at least five mil- confidence interval 1.23 to 1.65) and a mean serial interval lion view counts for the Ice Bucket Challenge on YouTube. for accepting the challenge of 2.1 days (median 1 day). Among the small pool of potentially eligible participants we Higher log (base 10) net worth of the participants was arbitrarily chose David Beckham, Cristiano Ronaldo, Ben- positively associated with transmission. edict Cumberbatch, Stephen Hawking, Mark Zuckerberg, CONCLUSIONS The Ice Bucket Challenge was moderately Oprah Winfrey, Homer Simpson, and Kermit the Frog as transmissible among a group of globally influential celebrities, index cases. We also included successful nominations up in the range of the pandemic A/H1N1 2009 influenza. The to the fifth generation seeded from each index case. challenge was more likely to be spread by richer celebrities, perhaps in part reflecting greater social influence. Data collection We completed a marathon viewing of 145 Ice Bucket Chal- Introduction lenges up to 13 September 2014. For each case we recorded The Ice Bucket Challenge, a campaign to raise awareness the age, sex, occupation, net worth, popularity (proxied by and support for people with amyotrophic lateral sclerosis,1 number of likes on Facebook and number of Twitter follow- is widely recognised to have gone socially viral. Although ers), number of successful nominations, and total number there are variations to the trial, participants typically of nominations. We obtained personal details of the celebri- complete the challenge (they are doused in ice water or ties from Wikipedia and specialised websites, although the make a donation) and nominate three others to undertake validity of such information cannot be reliably ascertained.11 it.2 As of 1 September 2014, more than 17 million videos Wikipedia has been used in previous studies12 13 and its com- related to the Ice Bucket Challenge were shared on Face- parative accuracy has been reported.14 From verified accounts book alone, and these were viewed more than 10 billion we obtained the number of Facebook likes and Twitter follow- times by more than 440 million people.3 Socially viral phe- ers. Where these were not available, we used the unofficial­ nomena include videos that are made popular by sharing accounts with the most likes or followers. We excluded on the internet, and a video is said to have gone viral if it unofficial­ accounts with fewer than 1000 likes or followers. spreads rapidly as a result of frequent sharing.4 5 However, We traced contacts through Google, YouTube, Facebook, the infectious disease modelling framework has seldom Twitter, and Instagram. Online written or video responses been used to quantify the transmissibility of such socially by the nominated contacts were used to determine com- viral phenomena.6 pletion of the challenge. For details of statistical analysis see thebmj.com. R0 (major determinant of size of epidemic) of Ice Bucket Challenge and other contagions Contagion Period of event/outbreak R0 Mean serial interval (days) Results Ice Bucket Challenge* 2014 1.43 2.1† Eight index cases complied with the inclusion criteria. In Pandemic influenza 2009 1.2-2.321 2.821 MERS-CoV 2012-14 0.5-1.322-24 9.6‡25 total we included 91 nominees up to the fifth generation Measles Recurring 14-1826 11.721 seeded from each index case, and a total of 99 participants Smallpox Before 1980, eradicated 4-107 17.721 were enrolled into the cohort. Overall, 24.2% of partici-

MERS-CoV=Middle East respiratory syndrome coronavirus. R0 values >1 represent a self sustaining epidemic. *Based on data pants had zero successful nominations, 32.3% had one, in current study only. †Median serial interval was 1 day. ‡Derived mean from fitted log normal distribution. 26.3% had two, and 17.2% had three. Among the index

28 20-27 December 2014 | the bmj MEDIA STUDIES

Transmissibility of the Ice Bucket Challenge among Jun Lei globally influential celebrities: retrospective cohort study Yuri & Julia Milner Evan Spiegel Shinya Yamanaka Michael Y Ni, Brandford H Y Chan, Gabriel M Leung, Eric H Y Lau, Herbert Pang Melinda Gates Julia Louis-Dreyfus Katie Couric Michael Bloomberg Stephen Colbert Sheryl Sandberg Sue Desmond-Hellmann Leigh Morgan Sam Hawgood

Marissa Mayer Kara Swisher Kyra Sedgwick Jennifer Beals Troian Bellisario

Detlef Schrempf Mark Zuckerberg Reed Hastings Macklemore Shawn Kemp Chris Hansen

Phil Spencer Elon Musk Markus Persson Cli Bleszinski

Neil Gaiman Chris Anderson Amanda Palmer Ben Folds Cara Delevingne Kara Swisher Selena Gomez Suki Waterhouse

James Franco Ryan Seacrest Michael Jordan

Individual R David Beckham Magic Johnson  Leonardo DiCaprio Tree structure of nominations associated with index case Mark Zuckerberg

Despite a lower R0 cases, Mark Zuckerberg’s nomination chain produced the reported in the press. Indeed this social viral pandemic than for measles or most successful number of contacts (total of 41) up to the shares a similar R0 with pandemic A/H1N1 2009 influenza, smallpox, the Ice fifth generation (figure). No serious adverse events arising or the high end of the Middle East respiratory coronavirus from the Ice Bucket Challenge act were observed in this since 2012 (table). Nine out of the 99 participants (9.1%) Bucket Challenge 16 spread quickly series, but adverse events have been reported elsewhere, completed the challenge but did not nominate anyone, including falls, head injuries, a temporomandibular joint analogous to self isolation. We included these participants across the world as dislocation, cuts, and at least one fatality. We estimated the in our analyses and assumed all losses to follow-up to have nominees became measure of transmissibility, R0, to be 1.43 (95% confidence declined the challenge. immediately interval 1.23 to 1.65) and a mean serial interval of accept- Possible factors accounting for the speed and extent “infectious” once ing the challenge of 2.1 days (median 1 day). We excluded of the Ice Bucket Challenge pandemic may include the nominated Homer Simpson and Kermit the Frog in the regression online social media mode of transmission and the short models because of difficulty in ascertaining their personal serial interval for taking up the challenge (table). Despite

characteristics. Participants with a higher log (base 10) net a lower R0 than for measles or smallpox, the Ice Bucket worth were more likely to spread the Ice Bucket challenge Challenge spread quickly across the world as nominees (odds ratio 1.63, 95% confidence interval 1.06 to 2.50), became immediately “infectious” once nominated. A pre- adjusted for age and sex. vious social network experiment found that individual uptake improved with reinforcing signals from clustered Discussion social ties.18 Therefore, specified nominations within a The Ice Bucket Challenge—a campaign to raise awareness social network, such as celebrities, may be more successful and support for people with amyotrophic lateral sclerosis in spreading promotional messages for public health inter- by dousing oneself with a bucket of iced water or giving a ventions than are generic nominations. However, whether donation—was moderately transmissible among a group such social networks should be leveraged for health com- 19 of globally influential celebrities. Our finding of an0 R value munication depends on the nature of the intervention. greater than unity (1.43), indicating sustained spread, in Competing interests and references are on thebmj.com. a celebrities based cohort is consistent with observations Accepted: 17 November 2014

the bmj | 20-27 December 2014 29 Copyright of BMJ: British Medical Journal is the property of BMJ Publishing Group and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.