Objectives: Elite professional football players and staff are a unique group that might give insight into the epidemiology of SARS-CoV-2 infections in Germany and thus can serve as a model for a geographical distribution and an estimation of undetected infections. Methods: In this prospective cohort study seroprevalence was determined twice in May and June 2020 in players and staff from German Bundesliga. As screening assays a commercial ELISA (Euroimmun) and a CLIA (Roche) was used and an in-house neutralisation assay (NT) as gold standard. Participants were tested twice weekly using PCR from nasopharyngeal and/or oropharyngeal swabs. Results: Seroprevalence (NT used as confirmation) in 2,164 samples from 1,184 players and staff was rather similar in May (23/1157 (1.99%)) and in June (21/1007 (2.09%)). All participants were PCR negative during the study period. Significant regional differences in seroprevalence were not observed. When comparing seroprevalence with the cumulative incidence of infections derived from the German notification system (subgroup matching to cohort; men, age: 20-69), IgG was found 8-10 times more frequently, pointing to a high rate of undetected infections. ELISA and CLIA correlated only moderately (Kappa 0.52). Conclusions: Seroprevalence with high quality diagnostic in Germany seemed to be around 2%. The number of undetected infections seems to be 8-10 times higher than notification data. Quality of antibody assays is rather variable, thus results should ideally be confirmed at least by a second assay to prove IgG positivity.
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