Clinical specimens
Twenty influenza virus-positive respiratory samples from patients with influenza-like illness were included. These samples were collected in the USA or in Finland during the 2007–2008 and 2008–2009 influenza seasons. Four groups of five specimens were selected to represent each of the seasonal influenza subtypes: A(H1N1) viruses, A(H3N2) viruses, influenza B viruses representing the Yamagata lineage and the Victoria lineage. Each original specimen was divided into 10 aliquots and stored at −80 °C until used for further experiments.