Objective: On June 9, 2009, the Thailand Ministry of Public Health received their first report of an outbreak of the pandemic A (H1N1) pdm09 that occurred in a school. The authors conducted a study to describe the epidemiological characteristics of the outbreak and its resurgence, estimate the basic reproduction number (R0) and review recommendations for prevention and control.
Material and Method: Active case finding in the school and reviewing reports to the national surveillance system identified 184 students infected by the new virus. A survey described the illness in the students and the prevention and control measures taken by the school. The basic reproduction number was estimated from data in the early epidemic phase. The other survey was done to assess factors contributing to the resurgence of the outbreak.
Results: Students with the pandemic A (H1N1) pdm09 had a mild illness resembling seasonal influenza. Overcrowding in the classroom and activities that mixed students from different classes contributed to transmission in the school. The basic reproduction number for this school-based setting is 3.58. The second outbreak occurred because of poor monitoring of absenteeism and management of ill students.
Conclusion: This was the first outbreak of the pandemic A (H1N1) pdm09 in Thailand. The source could not be identified. Effective control measures monitoring, screening, strict personal hygiene and proper management of ill students.

Keywords: Basic reproduction number, The pandemic A (H1N1) pdm09, School, Thailand