To highlight the importance of CD8 T cells in protecting against subsequent viral encounter, MHC class-II-deficient mice (MHC II −/− ), that lack CD4 T cells and therefore can not generate VACV-specific humoral immunity, were vaccinated with WR, Lister, and NYCBOH and challenged 70 days later with a lethal intranasal dose of VACV-WR (76) .