Biomolecular testing consisted, for three of the four centers, firstly in detection of influenza A virus by a home-made real-time RT-PCR (RT-PCR InflA) targeting the matrixprotein-coding gene and consequently by specific detection of the circulating pandemic variant using two monoplex real-time RT-PCR assays as described in the Centers of Disease Control (CDC) protocol: the SW InfA PCR (SWINE) and the SW H1 PCR (RT-PCR A/ H1N1) [20] . In the fourth center (26% of samples), a commercial available PCR kit was used directly for detecting the pandemic strain: "Swine influenza virus (sw H1N1) Real-time PCR" (Diagenode Diagnostics, Liège, Belgium).