Impact of the national test strategy on the development of the Covid-19 pandemic in Denmark

During the Covid-19 pandemic, Denmark has pursued a unique mass testing strategy culminating in testing of more than 8,000 citizens per 100,000 inhabitants per day in the Spring 2021. The strategy includes free access to Covid-19 testing and since 2021, compulsory documentation for negative test or vaccination for access to workplace, educational institutions, restaurants, and many other places. In the present study we analysed the effect of the Danish mass testing strategy throughout relevant stages of the pandemic. Mass testing was found not to have impact on the number of hospitalizations during the pandemic. Furthermore, during the highest level of testing in the spring 2021 the fraction of positive tests increased slightly on comparable days indicating that mass testing at best, did not reduce the prevalence of Covid-19. The relationship between positives in antigen testing and in PCR testing indicated that many patients are not tested early in their infection where the risk of transmission is highest. A likely explanation for the lack of impact of mass testing on hospitalizations and infections is that the testing increases risk-behaviour of the tested persons and that a high fraction of false negatives at low Covid-19 prevalence combined with increased risk-behaviour outweighs beneficial effects of mass testing.