Abstract
OBJECTIVES: Influenza can be a serious illness, especially for older people, and reducing the impact of influenza in elderly is important. The objective of this study was to estimate the prevalence and postinfection outcomes of influenza among the over-50 population in Japan. DESIGN: An observational study was designed to ascertain the proportion of influenza cases in a population aged ≥ 50 years with acute respiratory infection (ARI) and to determine the postinfection outcomes of their illness during the 2008-09 influenza season in Japan. Respiratory specimens obtained from a total of 401 patients were tested by PCR for influenza viruses, respiratory syncytial virus (RSV) and human metapneumovirus (hMPV). The effectiveness of the seasonal trivalent influenza vaccine was estimated by a test-negative case control analysis. SETTING: Seventeen outpatient clinics located in four separate areas of Japan. SAMPLE: Respiratory swab specimens from the ARI patients aged ≥ 50 years. MAIN OUTCOME MEASURES: Laboratory confirmed influenza in patients presenting with ARI. RESULTS: In all, 89 (22.2%) of the patients were positive for one of the tested viruses; 70 (78.7%) with influenza, 17 (19.1%) with RSV, and 2 (2.2%) with hMPV. Cough (95.7% vs 73.4%), loss of appetite (67.1% vs 35.5%), absence from work (50.0% vs 23.0%), impact on daily activity (90.0% vs 62.5%), and caregiver absence from work (5.7% vs 0.6%) were observed higher in influenza patients. The duration of feeling weakness (6.3 ± 5.4 vs 3.6 ± 1.9 days) and average days of reduced activity (5.2 vs 3.6 days) were longer for influenza patients. Vaccine effectiveness was estimated to be 32.1% (95% CI: -14.9, 59.9%). CONCLUSIONS: Influenza was the dominant ARI-causing virus and the clinical and socio-economic outcomes imposed on patients over 50 years of age was high for influenza.