Pandemic influenza A (H1N1) 2009: epidemiology, clinical features and differences with seasonal influenza in Chile

Pandemic influenza A (H1N1) 2009 raised questions regarding differences with seasonal influenza. Objectives: To describe the clinical features of pandemic influenza and compare them to seasonal influenza. Patients y Methods: A descriptive study that compared hospitalized adults was done between patients with confirmed pandemic influenza in the Hospital Clinico Universidad Catolica in Santiago, Chile, from May to July 2009 and 95 confirmed historic cases of seasonal influenza. Results: 54 patients with pandemic influenza were included, 51.9% were male, age of 52.8 +/- 19.5 years old; 79.6% had chronic diseases; 16.7% were immunocompromised patients and 7.4% of pregnant women. 25.9% of the patients acquired the infection during the hospitalization. 31.5% were admitted to intermediate/intensive care units. Pneumonia was diagnosed in 37%, and the mortality rate was 3.7%. The comparison between pandemic and seasonal influenza showed less proportion of patient >= 65 years of age (31.5% vs. 68%; p<0.0001); double number of nosocomial acquisition and more cases of pneumonia and death. Conclusions: The pandemic influenza infection affected younger people and was related with more nosocomial cases, pneumonia and mortality rates than seasonal influenza.