Bidirectional contact tracing dramatically improves COVID-19 control


Contact tracing is critical to controlling COVID-19, but most protocols only "forward-trace" to notify people who were recently exposed. Using a stochastic branching-process model, we show that "bidirectional" tracing to identify infector individuals and their other infectees robustly improves outbreak control, reducing the effective reproduction number ($R_\mathsf{eff}$) by at least ~0.3 while dramatically increasing resilience to low case ascertainment and test sensitivity. Adding smartphone-based exposure notification can further reduce $R_\mathsf{eff}$ by 0.25, but only if nearly all smartphones can detect exposure events. Our results suggest that with or without digital approaches, implementing bidirectional tracing will enable health agencies to control COVID-19 more effectively without requiring high-cost interventions.

medRxiv 2020.05.06.20093369