The B/Yamagata/16/88 (B/Yamagata, Yamagata lineage) virus was provided by Dr. Man Ki Song (International Vaccine Institute, Seoul, Korea). The B/Yamagata/16/88 virus was propagated in Madin-Darby canine kidney cells (ATCC CCL-34; ATCC, Manassas, VA, USA) as described previously (18 (link)19 (link)). The precipitated virus was suspended in PBS and stored at −80°C. For challenge infection, mice were lightly anesthetized with isoflurane and intranasally administered 5 to 10 LD50 of the virus.