Performance of the Danish contact tracing smartphone application, Smittestop during a year of the COVID-19 pandemic

<ns3:p> Objectives To assess the performance of the Danish digital contact tracing (DCT) application, Smittestop, during a one-year period of the COVID-19 pandemic. Methods The <ns3:italic>indicator framework for evaluating the public health effectiveness of DCT</ns3:italic> , developed by ECDC/WHO, is used as a guideline. Observational data from four different sources is presented and used to evaluate Smittestop. The study period ranges from November 22, 2020 to November 27, 2021. Results By November 27, 2021, Smittestop was downloaded 2,422,756 times in a population of 5,867,412 individuals (41.0%). 23.1% of SARS-CoV-2 positive individuals reported having Smittestop. The number of positive test results shared on Smittestop correlated to 23.1% of all PCR-confirmed cases. Out of 8.8 million answers to the test-indication questionnaire (34.2% of PCR-tested individuals), 117,171 reported a Smittestop-notification as their indication for testing. Of these, 1.0% (1,221) were test-positive. In comparison, 3.6% of known close contacts of infected individuals and 0.2% of individuals tested for screening purposes were positive. Extrapolations suggest that the number of identified cases would have increased substantially even with small increases in Smittestop coverage. Conclusion Smittestop identified cases better than mass screening, during a pre-Omicron period of the COVID-19 pandemic, but the number of identified cases is small when compared with the number of users. Improvements in the technology and higher adoption in the population could increase efficiency of Smittestop. </ns3:p>