Largely varying yield of universal SARS-COV-2 admission screening by population incidence

Introduction: A- or presymptomatic SARS-CoV-2 infected individuals pose a potential threat for health care facilities due to the risk of secondary cases. To prevent unnoticed introduction of SARS-CoV-2, several institutions implemented universal admission screening. However, little is known on the yield of this approach in different epidemiological settings. Objectives: We aimed to describe the rate of asymptomatic SARSCoV- 2 infected patients identified in universal admission screening, and its correlation with the population incidence. Methods: Publicly available cantonal data were used to describe the population incidence. All patients admitted to the University Hospital Zurich, Switzerland, were tested for SARS-CoV-2 by polymerase chain reaction (PCR). SARS-CoV-2 positive patients were retrospectively categorized as symptomatic or asymptomatic at admission and history for prior COVID-19 assessed by medical chart review. For correlation analyses, we calculated the positivity rates per calendar week for the admission screening and incidence rates per calendar week and 100'000 inhabitants for the cantonal data. Results: In the canton of Zurich, median SARS-CoV-2 incidence was 34.3 infections per calendar week and 100'000 inhabitants (interquartile range: 9.9 to 250.2). A total of 21'512 admissions screenings were performed in a 44-week period between calendar week 18 2020 and calendar week 8 2021. SARS-CoV-2 PCR was positive in 647 (3.0%) individuals. In 89 (13.8%) individuals, the positive test result was considered as residual viral replication after recent COVID-19 and not reflecting contagiousness. 457 (70.6%) SARS-CoV-2 positive individuals presented with COVID-19 symptoms at admission, whereas 101 (15.6%) SARS-CoV-2 individuals did not show any COVID-19 symptoms. Both, the rate of SARS-CoV-2 positive individuals (r = 0.88, P < 0.001) and the rate of asymptomatic SARS-CoV-2 positive individuals (r = 0.84, P < 0.001) correlated with the population incidence (Figure). Conclusion: In line with prior studies, approximately 0.5% of all admission screenings were positive in asymptomatic individuals. A strong correlation of the rate of asymptomatic SARS-CoV-2 infected individuals with the population incidence was detected.