Antigen testing was assessed by immunochromatographic rapid antigen detection (RAT) in three of the four centers or also by direct immunofluorescence (DIF) technique (Argene SA, Verniolle, France) in one of them. Both DIF and immunochromatography use highly sensitive monoclonal antibodies directed against either influenza A or B nucleoprotein antigens [18] . RAT was performed using two different testing: the Coris Influ-A&B Respi-Strip (Coris Bioconcept, Gembloux, Belgium) and the Binax Now influenza A & B (Binax Inc., Inverness medical, Maine, USA). Direct antigen testing was unavailable in the fourth participating hospital, representing 26% of our cohort of patients.