Overdispersion in action. In this church cluster, a single index case resulted in 53 (!) secondary blue cases. Yet, each blue case resulted in only a few tertiary cases. In fact, the median is 0 (note "35 additional people"). What are the implications for contact tracing? 1/3

This is an extreme example, but we see the value in tracing back to the source. There was something unique about this church service that resulted in many people being infected. Once a handful of infected church-goers were detected, it suggested there were more to be found. 2/3

Backwards tracing is about tracing orange to blue to church index case. Once back at the church, you have a better chance of finding new chains extending outwards. We may be too late to stop the orange cases, but can we prevent the green? Can we prevent the next cluster? 3/3