CD8A and infection: As suggested by CD8+ T cell data, assessment of lung viral titres showed delayed viral clearance on d6 after the secondary infection in case of heterologous prime-and-boost (PR8->X31-NPN3A and PR8-NPN3A->X31) compared to homologous infections (PR8->X31 or PR8-NPN3A->X31-NPN3A) (Fig. 5B).