Pandemic H1N1 illness prognosis: evidence from clinical and epidemiological data from the first pandemic wave in São Paulo, Brazil.

Objectives: The pandemic of 2009 H1N1 influenza A emerged in February 2009, with high morbidity and mortality, and rapidly spread globally. São Paulo was among the most affected areas in Brazil. This study compares the clinical and epidemiological characteristics of influenza-like illness between outpatients and hospitalized patients and evaluates the impact of oseltamivir therapy on the outcome of 2009 H1N1 influenza A patients.
Methods: This is a case series study comparing the clinical and epidemiological characteristics of influenza-like illness between outpatients attended at Hospital São Paulo in August 2009 (the peak of the first pandemic wave) and those patients hospitalized between May and September 2009 (the entire first pandemic wave).
Results: The 1651 patients evaluated were predominantly female (927×686, p<0.001) and aged 31.71±16.42 years, with 148 reporting chronic pulmonary disease. Dyspnea was presented by 381 (23.4%) patients and was more frequent among those aged 30 years or more (p<0.001). Hospitalization occurred at 3.73±2.85 days, and antiviral treatment started 2.27±2.97 days after the onset of first symptoms. A delay of more than 5 days in starting oseltamivir therapy was independently associated with hospitalization (p<0.001), a stay in the ICU (p<0.001) and a higher risk of dying (OR=28.1, 95% CI 2.81-280.2, p=0.007).
Conclusion: The 2009 pandemic of H1N1 influenza A affected young adults, presented a significant disease burden and produced severe cases with a significant fatality rate. However, promptly starting specific therapy improved the outcome.