Flowchart of case definitions (CD) for hospitalized respiratory infections. To be included as a case in the hospital-based surveillance of acute respiratory infections in Berlin, 2009/10, patients had to fulfil criteria of the broadest case definition (CD1). All CD1 who did not meet the CD4 (SARI) criteria were classified as CD1b. CD2 and CD3 fulfilled clinical criteria only and excluded those where clinical criteria were not fully met but a physician's (suspected) diagnosis of RI in the chart led to inclusion. The smallest subsample of CD1 were cases fulfilling CD4 criteria (SARI as defined by ECDC/WHO).