Lungs taken from mice after primary viral infection (Fig. S3A) or prime-and-boost approach using homologous (PR8-.X31 and PR8-NPN3A-.X31-NPN3A) or heterologous (PR8-.X31-NPN3A and PR8-NPN3A-.X31) strategy (Fig. 5B) were homogenised and the virus-containing supernatant above the cell debris was harvested and stored at 270uC. Titres of infectious virus in the lung supernatants were determined by plaque assay on monolayers of MDCK cells.