Fig. 2 |. Backward and frequency-based contact tracings are effective at reaching hub nodes.

a,b, We simulate the SEIR model on a BA network composed of 250,000 nodes. We sample the infected nodes with probability 0.1 and trace their parents at time t = 10. In a, the blue and orange lines respectively indicate the complementary cumulative distribution function (CCDF) for the degree of the sampled nodes and their parents, which follow G₁ and G₂, respectively. Frequency-based contact tracing (b)—isolating the most frequently traced nodes—can reach nodes with a degree similar to the parents without knowing who infects whom. c,d, As for the contact network, both backward (c) and frequency-based (d) contact tracing can reach large degree nodes for people-gathering networks. e, The bias due to backward tracing is present even in a relatively homogeneous network. We simulate the SEIR model on a temporal contact network of university students and sample all infected nodes and their parents. The infected and parent nodes have degree distributions that closely follow G₁ and G₂ for the unweighted aggregated network, respectively. f, As for the contact and people-gathering networks, frequency-based contact tracing is effective at reaching large degree nodes.