COVID-19 testing and cases - a complex relationship in three acts.
If and how the increasing numbers in confirmed COVID 19 cases are driven by the simultaneous expansion of test capacities and tests conducted is a question under continuous debate. Building on the hypothesis, that the impact of increased testing might not be constant over time but determined by the national test policy applied, the relationship between testing and number of cases was investigated in different phases of the pandemic with a focus on the time before and after a change in German legislation on May 22, 2020 that opened the door for more tests in asymptomatic persons for prevention and containment purposes. Based on linear regression models it is estimated that about 60% of the cases tested positive after May 22 can be attributed to increased testing, while eliminating the test effect does not substantially impact the numbers before that date. Likewise, the clinical presentation of cases registered after May 22 is significantly different, with a hospitalization rate of 8.12% (before 18.17%) and a case fatality rate of 0.63% (before 5.17%).
It is concluded that expanding the number of tests as it was done in Germany did not lead to a better understanding of the epidemiology of COVID 19. It is recommended to separate test results from preventive testing and testing based on a clinically defined test strategy, and to only use the latter for surveillance and as a basis for political decisions.