Bidirectional contact tracing could dramatically improve COVID-19 control

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

doi:10.1038/s41467-020-20325-7

Abstract

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 find that “bidirectional” tracing to identify infector individuals and their other infectees robustly improves outbreak control. In our model, bidirectional tracing more than doubles the reduction in effective reproduction number (Reff) achieved by forward-tracing alone, while dramatically increasing resilience to low case ascertainment and test sensitivity. The greatest gains are realised by expanding the manual tracing window from 2 to 6 days pre-symptom-onset or, alternatively, by implementing high-uptake smartphone-based exposure notification; however, to achieve the performance of the former approach, the latter requires nearly all smartphones to detect exposure events. With or without exposure notification, our results suggest that implementing bidirectional tracing could dramatically improve COVID-19 control.

Highlights

Contact tracing is critical to controlling COVID-19, but most protocols only “forward-trace” to notify people who were recently exposed
As a result of this fragility, our results suggest that digital exposure notification alone is unlikely to be a viable method for controlling COVID-19
Since manual bidirectional tracing with a 2-day window achieved a reduction in Reff of up to 0.48, these results suggest that implementing highuptake exposure notification alongside manual tracing could roughly double the efficacy of current contact tracing efforts at reducing COVID-19 transmission

Summary

Introduction

Contact tracing is critical to controlling COVID-19, but most protocols only “forward-trace” to notify people who were recently exposed. Assumed Previous reports suggest most contacts can be traced within one day, but some take much longer[47] Assumed effective bidirectional tracing or implementing high-uptake bidirectional digital exposure notification could substantially improve COVID-19 control.

Results

Conclusion