Low response to a monovalent inactivated unadjuvanted influenza A (H1N1) pdm09 vaccine in pediatricians of a general hospital in Japan.

Immunization of health care personnel (HCP) is critically important to reduce healthcare-associated influenza infections substantially. During 2009-2010, 74% of all HCP at Kitano Hospital, Osaka, Japan, including 94% of pediatricians, received the monovalent unadjuvanted influenza A (H1N1) pdm09 vaccine. We evaluated the vaccine's immunogenicity. Sixteen pediatricians received 15 μg hemagglutinin antigen subcutaneously. Antibody titer assays were conducted using hemagglutination-inhibition antibody assay on days 0 and 21, and at 5 mo after vaccination. Seroprotection rates, seroconversion rates, and geometric mean titer folds at 21 d were, respectively, 43.8%, 43.8%, and 5.4 in all subjects, 70.0%, 70.0%, and 8.0 in subjects aged 27-34 y, and 0.0%, 0.0%, and 8.0 in subjects aged ≥ 35 y. None of the latter group met the European Medicines Agency criteria. We hope to adopt intradermal routes and further the development of the influenza vaccine using new technology to improve immunogenicity in Japan.