That brings us to the question: How was a case identified in the first place? Scientists around Prof. Drosten from the Berlin Charité have developed the first test for SARS-CoV-2 and it was quickly accepted be the WHO to reliably detect infected individuals. The test itself is based on polymerase chain reaction (PCR) and detects the virus in individuals. In the meantime, several tests for SARS-CoV-2 are on the market, all based on PCR.
For valid and reliable test results, the collection method for the PCR test needs to be uniform and consistent for every sample. With little knowledge about the virus itself, there might have been a lack of understanding how exactly samples have to be collected in order to yield correct results. However, experts agree that the currently available PCR-tests are robust in detecting new cases.
Case definitions emerged over time, and in the end a distinction was made between “suspect case”, “probable case”, and “confirmed case” (1, 2). For example, the “suspect case” definition includes the presence of symptoms. However, especially in the beginning it was not clear what symptoms are associated with COVID-19 (and to which degree, i.e. mild vs. severe), and it became clear that there is the possibility for people to show no symptoms at all despite being infected with SARS-CoV-2. Moreover, the definition includes “history of travel to or residence in a location reporting community transmission of COVID-19 disease during the 14 days prior to symptom onset” (1). Again, especially in the beginning it wasn’t clear which locations were affected and the community transmission for some locations was only discovered in hindsight, so the definition of being a “suspect case” was subject to change. Having established a universal case definition has helped the comparability of case numbers across the globe, but this number still heavily relies on the reports from local health authorities and test capacities (needed to confirm a case), which are not equally distributed. Furthermore, the absolute case number does not take into account the underlying population size, so other measures needed to be found eventually.
As briefly mentioned, test capacities also played a major role in the beginning of the pandemic. They were restricted in the beginning and quickly ramped up to meet the necessary capacities after it became clear that it was crucial in fighting the pandemic. However, especially in the beginning, many countries lacked those capacities, either due to shortcomings in the available tests or because laboratories were not able to process the number of tests.
A few people raised concerns about the validity of reported case numbers, fueled by differing numbers reported by different sources. As an example, there were three sources that were most often cited when reporting case numbers in Germany, all reporting slightly different numbers, which especially in the beginning seemed like a big difference. The German Robert Koch-Institute (RKI) – which is the central institute of the German government for infectious and non-communicable diseases and its prevention – used the absolute case numbers reported from local health authorities, the European Center for Disease Control used numbers reported from its member states, and the often cited Johns-Hopkins-University relied on various open sources (3), making it the most up-to-date source especially in the beginning.