Abstract
BackgroundInfluenza pandemic preparedness involves priming of the population with pre-pandemic vaccines. Such vaccines should be well tolerated and induce a long-lasting immunological memory that can effectively be boosted with a single dose of pandemic vaccine once available. The presented studies assessed different prime-boost regimens with a Vero cell-derived whole virus non-adjuvanted H5N1 vaccine. MethodsIn one study, 281 healthy adult (18–59 years) and 280 elderly (≥60 years) subjects received two vaccinations, 21 days apart, with Vero cell-derived whole virus non-adjuvanted H5N1 vaccine (7.5μg HA antigen A/Vietnam/1203/2004) followed by a 6, 12–15, or 24 month booster (7.5 or 3.75μg A/Indonesia/05/2005 or A/Vietnam/1203/2004). In the other study, 230 healthy adults (18–59 years) received single dose priming (7.5μg A/Vietnam/1203/2004) followed by a 12 month booster (7.5 or 3.75μg A/Indonesia/05/2005). Antibody responses were assessed by microneutralization (MN) and single radial hemolysis (SRH) assay. Vaccine safety was assessed throughout. ResultsTwo dose priming was equally immunogenic in adults and the elderly: >72% of subjects in each population achieved MN titers ≥1:20 after the second vaccination. Booster vaccinations at 6, 12–15, and 24 months induced substantial antibody increases to both strains: after a 7.5μg A/Indonesia/05/2005 booster, 93–95% of adults and 72–84% of the elderly achieved MN titers≥1:20 against this strain. Homologous and heterologous booster responses were higher in the 7.5μg dose group than in the 3.75μg dose group. Booster responses following single dose priming were similar; a 7.5μg booster dose induced homologous MN titers ≥1:20 in 93% of subjects. ConclusionsA Vero cell derived whole virus non-adjuvanted H5N1 influenza vaccine is well tolerated and induces long-lasting cross-clade immunological memory that can be effectively boosted 1–2 years after two dose or single dose priming, supporting its suitability for pre-pandemic vaccination.
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