Swine-origin influenza a outbreak 2009 at Shinshu University, Japan

Abstract

Background: A worldwide outbreak of swine flu H1N1 pandemic influenza occurred in April 2009. To determine the mechanism underlying the spread of infection, we prospectively evaluated a survey implemented at a local university.

Methods: Between August 2009 and March 2010, we surveyed 3 groups of subjects: 2318 children in six schools attached to the Faculty of Education, 11424 university students, and 3344 staff members. Subjects with influenza-like symptoms who were diagnosed with swine flu at hospitals or clinics were defined as swine flu patients and asked to make a report using a standardized form.

Results: After the start of the pandemic, a total of 2002 patients (11.7%) were registered in the survey. These patients included 928 schoolchildren (40.0%), 1016 university students (8.9%), and 58 staff members (1.7%). The incidence in schoolchildren was significantly higher than in the other 2 groups (P < 0.0001) but there were no within group differences in incidence rate between males and females. During the period of the survey, three peaks of patient numbers were observed, in November 2009, December 2009, and January 2010. The first peak consisted mainly of schoolchildren, whereas the second and third peaks included many university students. Staff members did not contribute to peak formation. Among the university students, the most common suspected route of transmission was club activity. Interventions, such as closing classes, schools, and clubs, are likely to affect the epidemic curves.

Conclusion: Schoolchildren and university students are vulnerable to swine flu, suggesting that avoidance of close contact, especially among these young people, may be effective way in controlling future severe influenza pandemics, especially at educational institutions.

Figure 1

Location of institutions related to Shinshu University.

Figure 2

Serial changes in patient number from August 2009 to March 2010. a) Total numbers of patients per week of the study period. b) Serial changes in patient numbers for the 3 groups. Attached schools had summer recess in August, with school classes resuming in September 2009, whereas university classes had recess in August and September and restarted in October 2009. All subjects had winter recess from the end of December 2009 to the beginning of January 2010.

Figure 3

Cumulative incidence rates of patients in the 3 groups.

Figure 4

Epidemic curves and interventions. a) Epidemic curves and interventions of attached schools in districts A and F. Arrows indicates the periods of class closures (a and c) and a school closure (b). Each class closure ranged from 1 to 10 days, with some classes closed simultaneously. Kindergartens, elementary schools, and junior high schools in district A were closed simultaneously for 4 days, but only once. b) Epidemic curve and interventions of club closures (d) among university students. Club activity was stopped when more than two attendees were infected at the same time. Some clubs were closed simultaneously in October and November 2009, and the number of patients decreased after the interventions.