Lessons from the epidemiological surveillance program, during the influenza A (H1N1) virus epidemic, in a reference university hospital of Southeastern Brazil

introduction: the case definition of influenza-like illness (ili) is a powerful epidemiological tool during influenza epidemics. methods: a prospective cohort study was conducted to evaluate the impact of two definitions used as epidemiological tools, in adults and children, during the influenza a h1n1 epidemic. patients were included if they had upper respiratory samples tested for influenza by real-time reverse transcriptase polymerase chain reaction during two periods, using the ili definition (coughing + temperature > 38oc) in period 1, and the definition of severe acute respiratory infection (ars) (coughing + temperature > 38oc and dyspnoea) in period 2. results: the study included 366 adults and 147 children, covering 243 cases of ili and 270 cases of ars. laboratory confirmed cases of influenza were higher in adults (50%) than in children (21.6%) ( p < 0.0001) and influenza infection was more prevalent in the ili definition (53%) than ars (24.4%) (p < 0.0001). adults reported more chills and myalgia than children (p = 0.0001). oseltamivir was administered in 58% and 46% of adults and children with influenza a h1n1, respectively. the influenza a h1n1 case fatality rate was 7% in adults and 8.3% in children. the mean time from onset of illness until antiviral administration was 4 days. conclusions: the modification of ili to ars definition resulted in less accuracy in influenza diagnosis and did not improve the appropriate time and use of antiviral medication.