Tracing and testing multiple generations of contacts to COVID-19 cases: cost-benefit tradeoffs

Kim, J.; Chen, X.; Nikpey, H.; Rubin, H.; Bidokhti, S. S.; Sarkar, S.

Traditional contact tracing for COVID-19 tests the direct contacts of those who test positive even if the contacts do not show any symptom. But, by the time an infected individual is tested, the infection starting from the person may have infected a chain of individuals. Hence, why should the testing stop at direct contacts, and not test secondary, tertiary contacts or even contacts further down? One deterrent in testing long chains of individuals right away may be that it substantially increases the testing load, or does it? We investigate the costs and benefits of such multi-hop contact tracing for different number of hops. Considering a large number of contact topologies, spanning synthetic networks of divergent characteristics and those constructed from recorded interactions, we show that the cost-benefit tradeoff can be characterized in terms of a single measurable attribute, the initial epidemic growth rate. Once this growth rate crosses a threshold, multi-hop contact tracing substantially reduces the outbreak size compared to traditional contact tracing. Multi-hop even incurs a lower cost compared to the traditional contact tracing for a large range of values of the growth rate. The cost-benefit tradeoffs and the choice of the number of hops can be classified into three phases, with sharp transitions between them, depending on the value of the growth rate. The need for choosing a larger number of hops becomes greater as the growth rate increases or the environment becomes less conducive toward containing the disease.