Viral culture was performed on the three following cell lines: Vero, MRC-5 and LLC-MK2 in two centers; and MDCK, Hep-2, MRC-5 and LLC-MK2 in a third hospital, as described elsewhere [19] . In the fourth center which used DIF and RAT for antigen detection, respiratory samples were not cultured as antigen detection was followed directly by real-time PCR. (This center represented 22% of the cohort).