The samples retrieved from the deep-freeze archive were first generation isolates, i.e. the original clinical samples (NPAs) had been inoculated, for routine diagnostic testing, into Madin-Darby Canine Kidney (MDCK) cells. These MDCK-cultured viral isolates were originally confirmed to be influenza A(H3N2) before being frozen and archived. For this study, these frozen isolates were retrieved from deep-freeze, thawed and then used directly for sequencing. If any of these newly-thawed, archived samples failed to amplify at this stage, as determined by ethidium bromide staining and gel electrophoresis, it was inoculated into MDCK cells and re-cultured. After this additional step, most isolates were successfully sequenced. If this step still failed, then an alternative isolate was retrieved from deep-freeze for sequencing.