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Is testing at home sufficient? Self tests and official statistics

The question of how many new COVID-19 cases per week continues to navigate our political and personal decisions. How we assess the risk of infection, the methods we use to protect ourselves, and how we handle the pandemic in the future all depend on the answer to this question.

For several weeks now, Germany has been discussing a potential underreporting of its COVID-19 incidence rate. While more and more testing centers are closing due to lower summer demand, wastewater sample analysis and respiratory infection monitoring systems indicate increasing case rates. According to the current case definition, the reported 7-day incidence is based exclusively on PCR test results, while rapid antigen tests, whether performed professionally or at home, are not considered. Compared to rapid tests, PCR tests are often more accurate and ensure that local outbreaks can be detected correctly with a high degree of certainty. If the PCR test volume decreases, relative trends will still be visible, but the magnitude of new infections will be much more difficult to estimate.

Various research approaches are attempting to describe this degree of underreporting more precisely. Using questionnaire data from 28,274 data donors, in which we recorded test results and test types (e.g. PCR test, rapid antigen test), we aim to better estimate this situation by adding another piece to the puzzle.

What does our data show?

Our analysis consists of two steps: first, we compare how the official 7-day incidence differs from the “data donation” incidence based on your reported test results. Next, we examine PCR test rates over time to determine if a decline of this number is evident in our population and assess how it affects infection dynamics.

To get our analysis going, we first need to calculate the data donation incidence. For each day, we divide the number of self-reported positive tests via our App by the number of questionnaires submitted that day. From this, we calculate the 7-day average and multiply the result by 100,000. This enables us to estimate the number of cases per 100,000 donors and approximately compare our values to official statistics.