After the mass epidemic of the flu during the 2009/2010 season at a private junior and senior high school in Kanagawa Prefecture, a survey was conducted to investigate possible infection channels. The number of flu infections, initially limited to a few students belonging to extracurricular sports clubs during the 36th to 40th weeks of 2009, sharply increased following an all-school sports day event held in the 41st week, eventually affecting 14.4% of the entire student body. After the closing of 19 of 30 classes for 5 days of the same week in accordance to regional government standards, the number of affected individuals decreased at once, but subsequently increased following a cultural festival held in the 45th to 47th weeks, and infection rates reached 42.6% during the 2009/2010 season. However, there were no serious cases that resulted in any complications. Our investigations revealed that, of the two commuter bus routes, infection rates were significantly higher amongst students who used the busier, and more crowded, bus route compared to the less busy route. Across ages, junior high students contracted the flu at a significantly higher rate than high school students. In conclusion, our findings indicate that the standards defined by the regional government for the diagnosis and recognition of flu patients, and subsequent guidelines for school suspension were ineffective as far as infection prevention was concerned, possibly because asymptomatic and slightly symptomatic carriers were overlooked.
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