In contrast to the results seen in groups immunized with the non-adjuvanted H1N1 vaccine, TIV priming had no demonstrable effect on the immune responses generated against either the 0.3 μg or 3 μg HA formulated with AF03 adjuvant. Thus, at each time-point, no significant difference in HI titers was detected between the groups of unprimed and TIV-primed animals that received AF03-adjuvanted HDAC inhibitor vaccine (p > 0.07) The antibody results obtained using the HI assay were confirmed by SN tests performed on sera collected on Days 21 and 42 (Table 1). The results of these studies

conducted in BALB/c mice confirm and extend results obtained in human subjects showing that a single injection of pandemic influenza A (H1N1) 2009 vaccine formulated with or without an adjuvant is sufficient to induce HI antibody responses to protective levels in humans. An HI titer of 40 or more is generally considered to be associated with protection in humans against seasonal influenza [8] and [9]. In the present study, the geometric mean HI antibody titer generated against the pandemic (H1N1) 2009 influenza strain was

higher than 40 in all groups with the exception of the group of naïve mice immunized with 0.3 μg HA of non-adjuvanted vaccine. These results are in agreement with preliminary data reported from clinical trials that showed that a single dose of 15 μg HA of unadjuvanted pandemic (H1N1) 2009 vaccine is immunogenic and induced

antibody titers of 40 or more in 97% of subjects [10]. In the present study, the observed HI titers were higher than those reported by Dormitzer et al. who studied immune responses in naïve Ipatasertib clinical trial mice immunized with a subunit influenza vaccine [11]. However in Dormitzer’s study, serum was collected at earlier time-points after vaccination (Days 7 and 14) than in our study (Days 14 and 21) and the vaccinations were given 2 weeks apart, as compared to the 3-week interval heptaminol in our study. Differences in the immune responses observed in these two studies could also be explained by the composition and particulate structure of the vaccines used since subunit influenza vaccines have been reported to be less immunogenic in mice than split-virion vaccines [12]. Despite the inability of antibodies elicited by seasonal influenza vaccine to cross-react with the pandemic A/California/07/2009 (H1N1) strain, priming with seasonal influenza vaccines resulted in higher antibody responses to non-adjuvanted pandemic (H1N1) 2009 vaccine. These results are consistent with the results of clinical studies of the 1976 swine origin H1N1 influenza vaccine (A/New Jersey/76) in which that vaccine elicited low antibody responses in young subjects but significantly higher titers in older individuals, likely due to previous priming by vaccination or natural exposure to antigenically similar H1N1 influenza strains [13] before 1957.