Despite showing no neutralizing antibody responses, mice immunized with HA+NP+M2 were fully protected against lethal challenge. Lalor et al. showed a similar result in which a Vaxfectin-formulated DNA vaccine encoding H5+NP+M2 consensus genes protected mice against H5N1 challenge, despite low HAI responses. This is suggestive of other useful mechanisms of protection, such as cytotoxic T-lymphocytes, although this was not assessed in our study. A previous study with DNA/rAd5 immunization has also suggested that cellular immunity may contribute to protection in this model [12] .