![]() Therefore, digital contact tracing that leverages mobile devices may allow more swift and efficient contact tracing, potentially overcoming the limitations of traditional contact tracing 9. However, because traditional contact tracing is labour intensive and slow, its efficacy and cost–benefit trade-offs have been questioned 10, 11. Because voluntary testing and case isolation suffer from inevitable undetected transmissions, contact tracing is a potent intervention measure that allows the discovery and subsequent isolation of pre-symptomatic and asymptomatic cases, and plays a critical role for the successful control of emerging disease 3, 4, 5, 6, 7, 8, 9. Once the initial outbreak has been suppressed, it is critical to manage resurgence to avoid uncontrolled spreading and another lockdown. Mass quarantine has shown its effectiveness in controlling the epidemic outbreak during the coronavirus disease-2019 (COVID-19) pandemic, but with a considerable social and economic cost 1, 2. It is particularly crucial that we incorporate backward and deep tracing in a digital context while adhering to the privacy-preserving requirements of these new platforms. Our results call for a revision of current contact-tracing strategies so that they leverage all forms of bias. Using simulations on both synthetic and high-resolution empirical contact datasets, we show that strategically executed contact tracing can prevent a substantial fraction of transmissions with a higher efficiency-in terms of prevented cases per isolation-than case isolation alone. We argue that, even if the directionality of infection is unknown, it is possible to perform backward-aiming contact tracing. The effectiveness of backward tracing is due to simple but overlooked biases arising from the heterogeneity in contacts. Here we reveal that, compared with ‘forward’ tracing (tracing to whom disease spreads), ‘backward’ tracing (tracing from whom disease spreads) is profoundly more effective. However, what determines the efficacy of contact tracing has not been fully understood. Because many infectious diseases spread through interaction, contact tracing is widely used to facilitate case discovery and control. Effective control of an epidemic relies on the rapid discovery and isolation of infected individuals.
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