<p>　In order to clarify human mucosal and systemic immunity against influenza viral infection, the serum titers of anti-influenza virus-specific nasal secretory IgA and serum IgG and their changes after subcutaneous vaccination were measured in a Japanese healthy adult population in the present study. We recruited 155 healthy adults in 2006, with an average age of 24.1 years (range: 19-60 years). The male-female ratio was 1: 1. Nasopharyngeal lavage fluid and serum specimens were obtained prior to vaccination and a month after subcutaneous vaccination with a trivalent influenza ether split hemagglutinin vaccine of the A (H1N1), A (H3N2) subtypes and type B influenza viruses. Nasopharyngeal lavage fluid specimens were obtained by the nasal spray and aspiration method. The anti-influenza virus-specific IgA titers in the nasopharyngeal lavage fluid and IgG titers in the serum against the A (H1N1), A (H3N2) subtype and type B influenza viruses were measured by ELISA. The anti-influenza virus-specific nasal lavage fluid IgA titers were represented as a ratio to the total IgA titers. About 70% of the subjects had nasal anti-viral IgA against the A (H1N1), A (H3N2) subtype and type B influenza viruses, and almost all had serum anti-viral IgG against the A (H1N1), A (H3N2) subtype and type B viruses. The serum antiviral IgG titers, but not the nasal antiviral IgA titers, were significantly elevated at 1 month after the subcutaneous vaccination. Moreover, the serum antiviral IgG titers were significantly elevated after vaccination only in subjects with low pre-vaccination IgG titers, and not in those with high pre-vaccination IgG titers. The nasal antiviral IgA titers in the subjects of our present study were significantly higher than those in patients with influenza infection reported from our previous study. The presence of nasal anti-influenza virus-specific IgA in about 70% of Japanese adults is considered as being suggestive of a history of influenza infection. The presence of anti-influenza virus-specific IgG in the serum in almost all Japanese adults could suggest a history of influenza infection or influenza vaccination. Currently available subcutaneous influenza vaccines induce systemic immunity, with the appearance of anti-viral IgG in the serum, in adults. However, subcutaneous vaccination does not appear to be capable of inducing mucosal immunity with the induction of antiviral secretory IgA in the nasopharynx. The present findings suggest that subcutaneous influenza vaccination can suppress the progression of influenza infection by inducing the appearance of antiviral IgG in serum, but not by inducing the appearance of antiviral IgA in the nasopharynx. The findings also suggest that subjects with low antiviral secretory IgA titers in the nasopharynx are at a higher risk of influenza infection.</p>