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Pre-vaccination immunity and immune responses to a cell culture-derived whole-virus H1N1 vaccine are similar to a seasonal influenza vaccine . BACKGROUND Immune responses to novel pandemic influenza vaccines may be influenced by previous exposure to antigenically similar seasonal strains . METHODS An open-label , randomized , phase I/II study was conducted to assess the immunogenicity and safety of a non-adjuvanted , inactivated whole-virus H1N1 A/California/07/2009 vaccine . 408 subjects were stratified by age ( 18-59 and > 60 years ) and randomized 1:1 to receive two vaccinations with either 3.75 or 7.5 μg hemagglutinin antigen 21 days apart . Safety , immunogenicity and the influence of seasonal influenza vaccination and antibody cross-reactivity with a seasonal H1N1 strain was assessed . RESULTS A single vaccination with either dose induced substantial increases in H1N1 A/California/07/2009 hemagglutination inhibition ( HI ) and neutralizing ( MN ) antibody titers in both adult and elderly subjects . A single 7.5 μg dose induced seroprotection rates of 86.9 % in adults and 75.2 % in elderly subjects . Two 7.5 μg vaccinations induced seroprotection rates in adult and elderly subjects of 90.9 % and 89.1 % , respectively . The robust immune response to vaccination was confirmed by analyses of neutralizing antibody titers . Both HI and MN antibodies persisted for ≥ 6 months post-vaccination . Between 34 % and 49 % of subjects had seroprotective levels of H1N1 A/California/07/2009 antibodies at baseline . Higher baseline HI titers were associated with receipt of the 2008-09 or 2009-10 seasonal influenza vaccine . High baseline A/California/07/2009 neutralizing antibody titers were also associated with high baseline titers against A/New Caledonia/20/99 , a seasonal H1N1 strain which circulated and was included in the seasonal vaccine from 2000-01 to 2006-07 . Pre-adsorption with A/H1N1/New Caledonia/20/99 antigen reduced A/H1N1/California/07/2009 baseline titers in 55 % of tested sera . The vaccine was well tolerated with low rates of fever . CONCLUSIONS A whole-virus H1N1 A/California/07/2009 vaccine was safe and well tolerated and a single dose induced substantial immune responses similar to seasonal influenza vaccines , probably due to immunological priming by previous seasonal influenza vaccines or infections .
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Intervention_Pharmacological
inactivated whole-virus H1N1 A/California/07/2009 vaccine
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Outcome_Physical
seroprotection rates
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Outcome_Physical
seroprotective levels of H1N1 A/California/07/2009 antibodies
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Outcome_Physical
Higher baseline HI titers
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Outcome_Physical
High baseline A/California/07/2009 neutralizing antibody titers
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Outcome_Other
tolerated