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

Contact tracing constitutes the backbone of nonpharmaceutical public interventions against COVID-19, as it did with previous pandemics. Experts argue that its importance rises again as vaccination rates increase and the spread of COVID-19 slows, which makes tracing of individual cases possible. However, because randomized experiments on contact tracing are infeasible, causal evidence about its effectiveness is missing. This shortage of evidence is alarming as governments around the world invest in large-scale contact tracing systems, frequently facing a lack of cooperation from the population. Exploiting a large-scale natural experiment, we provide evidence that contact tracing may be even more effective than indicated by previous correlational research. Our findings inform current and future public health responses to the spread of infectious diseases.