The German Corona Warning App (CWA) first appeared in Google and Apple stores a year ago. Now the Federal Ministry of Health (BMG) and the Robert Koch Institute (RKI) have taken stock. The state contact tracing tool was never conveyed as a savior, emphasized a BMG spokesman. However, it makes an important epidemiological contribution to containing the pandemic, as a preliminary evaluation has now shown for the first time.
Contact tracking with the CWA is anonymous and decentralized. However, the RKI has succeeded in collecting some data in the past few months in order to be able to better assess the effectiveness of the system. On the one hand, since release 1.13 from March 4, 2021, users can voluntarily donate data, for example for risk calculations and to release their positive test warning. At the beginning of June, according to the BMG, more than 8 million users were already using it.
On the other hand, a voluntary online survey was built into the CWA, to which the app invited users who had received a red warning (“increased risk”). This was divided into a basic survey and a follow-up survey five days later. The campaign ran from March 4 to May 27, 2021. More than 15,000 users took part in both surveys.
Two-thirds surprised by warning
According to the evaluation, around 88 percent of those surveyed stated that they had activated the risk determination during the last 14 days. About two thirds were surprised by the red warning. In the first survey, 65 percent said they wanted to do a CoV-2 test. A total of 6 percent of the participants actually tested positive in the follow-up survey (this value corresponds to the so-called Secondary Attack Rate SAR). 80 percent of those tested positive shared their results directly in the CWA to warn their environment.
The RKI combined the data from the survey with the information obtained from the data donations in order to extrapolate the effectiveness of the app, and illustrated the results with an example: Were 4000 positive tests from users fed in on a certain day in the third wave , showed that between 20,000 and 40,000 CWA users had been warned. Of these, 16,000 to 32,000 could have been tested again, which would have revealed an additional 1,000 to 2,000 infections.
Up to 230,000 people tested positive by the CWA
The RKI extrapolates these estimates to all 475,000 positive tests reported in the CWA to date. This would result in 2.4 to 4.8 million red warnings and 1.9 to 2.8 million tests performed. According to these estimates, a total of 110,000 to 230,000 CWA users tested positive after a red warning and thus pulled out of chains of infection.
According to a paper by BMG and RKI, this corresponds “approximately to the positive rate that can be observed in analog contact tracking”. In other words: The RKI estimates the contribution that the CWA makes to containment to be roughly as high as that of all health authorities together.
BMG and RKI emphasize that this is an initial, nonetheless scientific assessment. The detailed written evaluation should follow in autumn 2021.