The Rules of Contagion

If we want to predict the shape of an outbreak, there are two things
we really need to know: how many additional infections each case
generates on average (i.e. the reproduction number), and the lag
between one round of infection and the next (i.e. the generation time).
During new disease outbreaks, we rarely know these values, so we
have to try and estimate them. For neknomination, though, the
information was laid out as part of the game. Each person nominated
2–3 others, and these people had to do the challenge – and make
their nominations – within 24 hours. When I forecast the
neknomination game in 2014, I didn’t have to estimate anything; I
could plug the numbers straight into a simple disease model.[63]
My outbreak simulations suggested that the neknomination trend
wouldn’t last long. After a week or two, herd immunity would kick in,
causing the outbreak to peak and begin to decline. If anything, these
simple forecasts were likely to overestimate transmission. Friends
tend to cluster together in real life; if multiple people nominate the
same person during the game, it will reduce the reproduction number
and lead to a smaller outbreak. Interest in neknomination indeed
faded quickly. Despite the UK media frenzy in early February 2014, it
was all but gone by the end of the month. Subsequent social media
games followed a similar structure, from ‘no makeup selfie’ photos to
the widely publicised ‘ice bucket challenge’. Based on the rules of the
games, my model predicted all of them would peak within a few
weeks, just as they did in reality.[64]
Although nominated-based games have tended to fade away after
a few weeks, social media outbreaks don’t always disappear after
their initial peak in popularity. Looking at popular image-based
memes on Facebook, Justin Cheng and his collaborators have found
that almost 60 per cent recurred at some point. On average, there
was just over a month between the first and second peaks in
popularity. If there were only two peaks, the second cascade of
sharing was generally briefer and smaller; if there were multiple
peaks, they were often a similar size.[65]
What makes a meme become popular again? The team found that
a big initial peak in interest made it less likely that the meme will
appear again. ‘It is not the most popular cascades that recur the