Evaluation of intrahospital test data as an indicator of SARS-CoV-2 incidence underreporting

Abstract The incidence has been widely used to assess the epidemiological situation during the COVID-19 pandemic and guide health policy. As testing requirements changed during the pandemic, more SARS-CoV-2 infections may have remained undetected. This study aims to evaluate the use of different indicators for estimating unreported SARS-CoV-2 infections. Monthly SARS-CoV-2 incidences in the general Bavarian population were compared with three indicators: incidence in healthcare workers (HCWs), incidence of patients, and incidence in visitors. Based on this, the number of unreported SARS-CoV-2 cases in the Bavarian population was estimated. The incidence from the visitors correlated moderately with the official incidences (r = 0.63). The general population incidence and the inpatient incidence are highly correlated (r = 0.91), as are the general population incidence and the HCWs incidence (r = 0.94). Between April 2020 and June 2023, the general population incidence was on average higher than the average incidence of the three indicators, after which the relationship reversed. The reversal of the relationship between the data series of the Bavarian population and the chosen indicators in June 2022 suggests that SARS-CoV-2 infections were underreported. Inpatient screening incidence and HCWs’ incidence in settings with low-threshold test accessibility are suitable indicators of current epidemiology in the general population.