small. The vast majority consisted of a single shooting, with no
additional contagion. However, occasionally the outbreaks were much
larger; one included almost five hundred linked shootings. When we
see these highly variable outbreak sizes, it suggests that
transmission is driven by superspreading events. Analysing the
outbreak data from Chicago in more detail, I estimated that
transmission of gun violence was highly concentrated. It’s likely that
fewer than 10 per cent of shootings led to 80 per cent of follow-up
attacks.[23] Just like disease transmission – which can be similarly
influenced by superspreading – most shootings didn’t lead to any
additional contagion.
The chains of transmission in Chicago also revealed the speed of
transmission. On average, the generation time between one shooting
and another was 125 days. Despite the attention given to dramatic
retaliations like the Brighton Park attack in May 2017, it seems there
are a lot of slower-burning feuds out there that have historically gone
undetected.
These networks of shootings help explain why the Cure Violence
approach is possible. Let’s start with the fact that we can study the
networks at all: if we want to control an outbreak, it helps if we can
identify potential routes of transmission. Slutkin has compared
violence interruption to the methods used to control smallpox
outbreaks. As smallpox was nearing eradication in the 1970s,
epidemiologists used ‘ring vaccination’ to stamp out the final few
sparks of infection. When a new disease case appeared, teams
would track down people the infected may have come into contact
with, such as family members and neighbours, as well as these
people’s contacts. They would then vaccinate people within this ‘ring’,
preventing the smallpox virus spreading any further.[24]
Smallpox had two features that worked in health teams’ favour. To
spread from one person to another, the disease generally required
fairly long face-to-face interactions. This meant teams could identify
who was most at risk. In addition, the generation time for smallpox
was a couple of weeks; when a new case was reported, teams had
enough time to go and vaccinate before more cases appeared. The
spread of gun violence shares these features: violence is often