Comparisons of Clinical Characteristics between Adult Patients with 2009 H1N1 Influenza and Those with Seasonal Influenza during the 2009 Epidemic in Thailand

Clinical presentations of patients with 2009 H1N1 influenza are generally similar to those of patients with seasonal influenza. A cross-sectional study was conducted among adults at an outpatient clinic in a university hospital setting during the 2009 epidemic. Infections in all patients were confirmed by real-time reverse transcription-polymerase chain reaction. There were 269 patients with 2009 H1N1 influenza and 128 patients with seasonal influenza. Compared with patients with seasonal influenza, patients with 2009 H1N1 influenza were more likely to be aged <20 years (25.7% vs. 8.6%, P < 0.001) and to be students (38.7% vs. 22.0%, P = 0.001), or healthcare workers (20.1% vs. 10.9%, P = 0.022); however, they were less likely to have an underlying disease (25.6% vs. 39.8%, P = 0.004) and influenza vaccination (4.2% vs. 12.2%, P = 0.021). There were no differences in the illness duration, vital signs, and symptoms between groups. Patients with seasonal influenza were more likely to be admitted to the intensive care unit (2.4% vs. 0%, P = 0.033) and exhibited a higher mortality rate (2.4 % vs. 0%, P = 0.093). A history of contact with a confirmed 2009 H1N1 influenza case (odds ratio [OR] = 12.91, P < 0.001) and age (OR = 1.05 per 10 years younger, P = 0.015) were associated with 2009 H1N1 influenza infections. In conclusion, these results showed differences in characteristics between the patients with 2009 H1N1 influenza and those with seasonal influenza.