Analysis of the influenza A (H1N1) 2009 pandemic infection in Japanese asthmatic patients: using a questionnaire-based survey.

Background: Influenza infection is known to be an exacerbating factor in the control of asthma, therfore its prevention is critical in managing asthma. The aim of this study was to investigate the influenza A H1N1 2009 pandemic virus (H1N1 pdm09) infection in adult asthmatic patients. Methods: Data were obtained from a questionnaire-based survey of asthmatic patients conducted from September to October 2010 in Niigata Prefecture. Patient background, H1N1 pdm09 infection, vaccination status, and asthma exacerbation due to influenza infection were analyzed. Results: In total, 2,555 cases were analyzed. The incidence of the infection was 6.7% (95% confidence interval [Cl]: 5.7-7.6), and the rate of vaccination was 63.9% (95% CI: 62.1-65.8). The odds ratio (OR) for vaccination against the infection among adult patients and younger patients (<= the median age) were 0.61 (95% Cl: 0.45-0.84) and 0.62 (95% Cl: 0.42-0.90), respectively. However, OR among the older patient (> median age) were 1.38 (96%Cl: 0.66-2.89). The rate of infection-induced asthma exacerbation was 23.2% (95% CI: 18.6-29.6), and the OR for vaccination against the infection-induced asthma exacerbation was 1.42 (95% Cl: 0.69-2.92). Conclusions: The effectiveness of the vaccination against the H1N1 pdm09 virus was confirmed during the first pandemic season, but it was limited. Further investigation on H1N1 pdm09 virus infection in asthmatics will be required.