Introduction In Japan, the first infected person to the influenza(A/H1N1) was confirmed, May 2009, and spread widely, especially among children. Schools conducted various preventive actions under a general guideline by the local education board. School or class closure was one of them, which was decided by each school. However, it is not clear whether school closure is effective. The objective of the study was to examine the effectiveness of school closure.

Methods Data were collected from June 2009 to February 2010, from all elementary and junior high schools located in a northern sub-prefecture of Ibaraki, 100 km north of Tokyo (approximate population of 280 000). The information included school name, school address, the enrolment number by school, dates of absentees from influenza-like illness (type-A), date of class closure and date of school closure. Impacts of school closure was analysed considering timing of closure, enrolment number, geographical location.

Result All 68 schools responded for study. 8576 out of 23 880 (143 280 person-months) enrolled school children were infected. We classified these schools into two groups according to date of school closure. The difference of incidence between two groups was estimated with the RR with 95% CI. The incidence rates in the early and late school closure groups were 71.6 and 64.9 (1/1000 person-months), respectively. RR was 1.10 (95% CI 1.05 to 1.15), which was significant.

Conclusion The result would show that the early school closure may have impact to prevent infection number of A/H1N1 influenza.