Epidemiological investigation of an outbreak of pandemic influenza A (H1N1) 2009 in a boarding school: serological analysis of 1570 cases

Abstract

Background: A large number of 2009 pandemic influenza A (H1N1) infections were localized in school populations.

Objectives: To describe the epidemiology, clinical features and risk factors associated with an outbreak that occurred at a vocational boarding school in Guangzhou, P.R. China.

Study design: Data were collected prospectively and retrospectively through the use of on-site doctors and a post-outbreak survey and blood collection. The survey was used to confirm symptoms, and to investigate a series of flu-related factors such as dormitory conditions, health habits, vaccine history and population contact history. Blood samples were taken for serological analysis. Pandemic H1N1 infection was initially confirmed by a real-time RT-PCR assay. Following the identification of the outbreak by the Guangzhou CDC on September 4, cases were diagnosed symptomatically and retrospectively by serological analysis using the hemagglutination inhibition assay and a neutralization assay.

Results: The infection rate was 32% (505/1570) and the attack rate was 22.2% (349/1570). The asymptomatic infection rate was 9.9% (156/1570). Sharing a classroom (OR=2.17, 95% CI: 1.62-2.91) and dormitory space (OR=2.32, 95% CI: 1.84-2.93) was associated with higher rates of infection. Opening windows for ventilation was the only control measure that significantly protected against infection.

Conclusion: Social isolation and quarantine should be used to prevent the spread of infection. Ventilation and a control of air flow between classrooms and dorms should be implemented as possible. School closures may be effective if implemented early.