Respiratory samples (nasopharyngeal aspirate or nasopharyngeal swab or tracheal secretion) from all three hospitals were routinely sent to the Institute of Virology and Immunobiology of the University of Wuerzburg and analyzed using reverse transcriptase-polymerase-chain -reaction (RT-PCR) or direct immunofluorescence (DIF) testing for influenza A and B. All specimens tested positive for influenza by DIF were determined by PCR for subtype determination.