For example, two reassortant influenza vaccines expressing H5 or H7 HAs (PR8-H5-H7NA and PR8-H7-H5NA) have been shown to replicate to high titers when exposed to exogenous neuraminidase (NA) in vitro, and these vaccine candidates were shown to be replication-defective and nonvirulent when administered intranasally in mice [19]. Here, XK is linked to influenza.