{IntroductionRecent} studies have demonstrated that inactivated seasonal influenza vaccines ({IIV)} may elicit production of heterosubtypic antibodies, which can neutralize avian {H5N1} virus in a small proportion of subjects. We hypothesized that prime boost regimens of live and inactivated trivalent seasonal influenza vaccines ({LAIV} and {IIV)} would enhance production of heterosubtypic immunity and provide evidence of cross-protection against other influenza {viruses.MethodsIn} an open-label study, 26 adult volunteers were randomized to receive one of four vaccine regimens containing two doses of 2009-10 seasonal influenza vaccines administered 8 (±1) weeks apart: 2 doses of {LAIV;} 2 doses of {IIV;} {LAIV} then {IIV;} {IIV} then {LAIV.} Humoral immunity assays for avian {H5N1}, 2009 pandemic {H1N1} ({pH1N1)}, and seasonal vaccine strains were performed on blood collected pre-vaccine and 2 and 4 weeks later. The percentage of cytokine-producing T-cells was compared with baseline 14 days after each {dose.ResultsSubjects} receiving {IIV} had prompt serological responses to vaccine strains. Two subjects receiving heterologous prime boost regimens had enhanced haemagglutination inhibition ({HI)} and neutralization ({NT)} titres against {pH1N1}, and one subject against avian {H5N1;} all three had pre-existing cross-reactive antibodies detected at baseline. Significantly elevated titres to {H5N1} and {pH1N1} by neuraminidase inhibition ({NI)} assay were observed following {LAIV-IIV} administration. Both vaccines elicited cross-reactive {CD4+} T-cell responses to nucleoprotein of avian {H5N1} and {pH1N1.} All regimens were safe and well {tolerated.ConclusionNeither} homologous nor heterologous prime boost immunization enhanced serum {HI} and {NT} titres to 2009 {pH1N1} or avian {H5N1} compared to single dose vaccine. However heterologous prime-boost vaccination did lead to in vitro evidence of cross-reactivity by {NI;} the significance of this finding is unclear. These data support the strategy of administering single dose trivalent seasonal influenza vaccine at the outset of an influenza pandemic while a specific vaccine is being {developed.Trial} {RegistrationClinicalTrials.gov} {NCT01044095}