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.