Sideward contact tracing and the control of epidemics in large gatherings

Mancastroppa, Marco; Guizzo, Andrea; Castellano, Claudio; Vezzani, Alessandro; Burioni, Raffaella

Effective contact tracing is crucial to contain epidemic spreading without disrupting societal activities especially in the present time of coexistence with a pandemic outbreak. Large gatherings play a key role, potentially favoring superspreading events. However, the effects of tracing in large groups have not been fully assessed so far. We show that beside forward tracing, which reconstructs to whom disease spreads, and backward tracing, which searches from whom disease spreads, a third"sideward"tracing is at work in large gatherings. This is an indirect tracing that detects infected asymptomatic individuals, even if they have neither been directly infected by, nor directly transmitted the infection to the index case. We analyze this effect in a model of epidemic spreading for SARS-CoV-2, within the framework of simplicial activity-driven temporal networks. We determine the contribution of the three tracing mechanisms to the suppression of epidemic spreading, unveiling the importance of sideward tracing and suggesting optimal strategies. We show the practical relevance of our results on an empirical dataset of gatherings in a University Campus.