Empirical evidence on the efficiency of bidirectional contact tracing in COVID-19

Abstract Despite ubiquitous rollout of contact tracing to counteract the spread of COVID-19, few countries have been spared from widespread community transmission, highlighting the need for more effective tracing strategies1,2. Standard contact tracing practice identifies, quarantines and tests persons exposed to an infected person during the contagious period, assumed to start two days before symptom onset or diagnosis3,4. Backward contact tracing intends to identify the source of the infection and persons infected by the same source, either by extending the contact tracing window or investigating suspected source events. These approaches have shown promise in modelling studies, but lack empirical data supporting their efficiency5–7. In the first large cohort study on backward contact tracing for COVID-19, we found that extending the contact tracing window backward by 5 days increased the number of identified contacts by 49.2%. The risk of infection amongst these additional contacts was similar to contacts exposed during the standard tracing window and significantly higher than symptomatic individuals in a control group, leading to an increase of 42.0% in cases identified through contact tracing. The risk was not limited to attendees of suspected source events. Our results imply an urgent need to implement backward contact tracing globally.