Background. Direct comparisons of similar doses of a novel influenza virus antigen administered by the intradermal route and the intramuscular route have not been reported.Methods. A total of 227 healthy adults aged 18-49 years were randomized to receive 2 doses 1 month apart of a subvirion inactivated influenza A virus subtype H5N1 (rgA/Vietnam/1203/2004) vaccine containing 38.7 μg of H5N1 hemagglutinin (HA), by the intramuscular route or by the intradermal route using the Mantoux technique. Clinical and serologic responses were assessed.Results. Injection site reactions were more frequent in the intradermal group. Immune responses and the geometric mean titer of serum hemagglutination inhibition and neutralizing antibodies 1 month after receipt of the first dose were similar and low but were significantly higher after 2 doses of vaccine in both groups.Conclusions. Intramuscular and intradermal delivery of vaccine were both well tolerated. Immune responses after 2 doses of this influenza A/H5N1 HA (38.7 μg) were low and not significantly different when given by the intradermal or intramuscular route. Evaluation of higher dosages, alternative intradermal delivery methods, and the addition of adjuvants will be needed to enhance the immunogenicity of inactivated influenza A/H5N1 vaccines by the intradermal route.Clinical Trials Registration. NCT00439335.
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