Measuring the scientific effectiveness of contact tracing: Evidence from a natural experiment

Contact tracing has for decades been a cornerstone of the public health approach to epidemics, including Ebola, severe acute respiratory syndrome, and now COVID-19. It has not yet been possible, however, to causally assess the method's effectiveness using a randomized controlled trial of the sort familiar throughout other areas of science. This study provides evidence that comes close to that ideal. It exploits a large-scale natural experiment that occurred by accident in England in late September 2020. Because of a coding error involving spreadsheet data used by the health authorities, a total of 15,841 COVID-19 cases (around 20% of all cases) failed to have timely contact tracing. By chance, some areas of England were much more severely affected than others. This study finds that the random breakdown of contact tracing led to more illness and death. Conservative causal estimates imply that, relative to cases that were initially missed by the contact tracing system, cases subject to proper contact tracing were associated with a reduction in subsequent new infections of 63% and a reduction insubsequent COVID19-related deaths of 66% across the 6 wk following the data glitch.

Automated summary

This study examines a large-scale natural experiment in England where a coding error led to a breakdown in contact tracing, resulting in more illness and death. Conservative estimates suggest that proper contact tracing following the data glitch was associated with a significant reduction in subsequent new infections and COVID-19-related deaths.