Contact tracing is crucial for COVID-19 control, but most protocols only forward-trace. This study uses a stochastic branching-process model to demonstrate that bidirectional tracing, identifying both infectors and infectees, significantly improves outbreak control. Bidirectional tracing more than doubles the reduction in the effective reproduction number (R_eff) compared to forward-tracing alone, showing greater resilience to low case ascertainment and test sensitivity. Expanding the temporal tracing window or implementing high-uptake smartphone-based exposure notification enhances effectiveness. The findings suggest that bidirectional tracing could dramatically improve COVID-19 control.

William J. Bradshaw, Ethan C. Alley, Jonathan H. Huggins, Alun L. Lloyd, Kevin M. Esvelt