Prospective Epidemiological Evaluation of Seasonal Influenza in All Elementary Schoolchildren in Matsumoto City, Japan, in 2014/2015.

Seasonal influenza is known to spread within and among educational organizations. Detailed understanding of the pattern of infection requires comprehensive prospective epidemiological studies, involving all schools within a community. This prospective survey evaluated 13,217 schoolchildren attending all of the 29 public elementary schools in Matsumoto City, Japan, in 2014/2015. Questionnaires were distributed to school nurses to obtain information about onset date and suspected route of transmission of influenza for all schoolchildren diagnosed with influenza virus at medical institutions. Responses were obtained for 2,548 infected schoolchildren, representing 96% of reported cases. Epidemic curves were plotted for each school by calculating the numbers of incident cases. Distance between schools was not associated with influenza spread over time. However, modeling showed that the occurrence of initial infection at each school and its spread over time could be fitted with a logistic curve. The transmission route for most children initially infected at each school was through a household member, whereas for most remaining schoolchildren it was through the school. These findings indicated that seasonal influenza was initially transmitted to schoolchildren by household members and then spread throughout the schools, with the initially infected child at each school fitting logistic curves over time.