[Epidemiological study of A (H1N1) pdm09 in Iki Island, Nagasaki].

An epidemic situation of pandemic (H1N1) 2009 was investigated from August 1 to March 31 in Iki City, an isolated island with a population of about 30,000. All members of the Iki Physicians' Association participated in the investigation. Daily reports of patients with influenza were made to the local health center. During the outbreak, 2,024 individuals were clinically diagnosed as having influenza, or with the rapid diagnostic test. The prevalence was highest among patients 10-19 years of age (26.8%), followed by patients 9 years and under (21.3%). The prevalence was lowest in patients over 60 years (0.4%). Of the 2,024 patients, 1,443 (71.3%) were 19 years of age or younger. Only seven of the 2,024 were admitted to the hospital, and there were no deaths. When the first outbreak occurred, many individual classes and some schools were closed, and subsequently the number of cases rapidly decreased. A second outbreak occurred three weeks later. The same procedures were taken and the outbreak came to an end. A vaccination program for A (H1N1) pdm09 virus was implemented according to the Japanese National Ministry of Health guidelines, one time for persons aged 13 years or over and twice for those 12 years or younger. To test for antibodies to pandemic A (H1N1) pdm09 after the outbreak, 358 serum samples were collected from Aug to Nov 2009, and the HI titer was measured. 205 (57.3%) were HI titer > or = 1:40. The factor most closely related to a 40 HI titer > or = 1:40 was A (H1N1) pdm09 vaccination, followed by symptomatic influenza. Asymptomatic influenza accounted for 11%. In conclusion, 71.3% of 2,024 A (H1N1) pdm09 patients were 19 years of age or younger. From an epidemiological perspective, school and class closures were considered to be very effective in controlling the spread of the disease.