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Concurrent comparison of epidemiology, clinical presentation and outcome between adult patients suffering from the pandemic influenza A (H1N1) 2009 virus and the seasonal influenza A virus infection
  1. Kelvin K W To,
  2. Samson S Y Wong,
  3. Iris W S Li,
  4. Ivan F N Hung,
  5. Herman Tse,
  6. Patrick C Y Woo,
  7. Kwok-Hung Chan,
  8. Kwok-Yung Yuen
  1. Infectious Disease Division, Queen Mary Hospital, State Key Laboratory of Emerging Infectious Diseases, Carol Yu Centre for Infection, The University of Hong Kong, Hong Kong Special Administrative Region, China
  1. Correspondence to Professor Kwok-Yung Yuen, Carol Yu Centre for Infection and Division of Infectious Diseases, Department of Microbiology, The University of Hong Kong, Queen Mary Hospital, Pokfulam Road, Pokfulam, Hong Kong Special Administrative Region, China; kyyuen{at}


Purpose of study The demographics, clinical features and outcome of patients with pandemic influenza A (H1N1) 2009 infection were compared with a concurrent cohort of patients with seasonal influenza A infection.

Study design The clinical and microbiological data of hospitalised adult patients admitted between 29 June and 28 October 2009, with pandemic A (H1N1) 2009 or seasonal influenza A infection, were analysed.

Results A total of 186 patients including 69 pandemic A (H1N1) and 117 seasonal influenza were analysed. The majority (75%) under 50 years of age had pandemic A (H1N1). Compared with seasonal influenza, pandemic A (H1N1) patients were younger (median age 47 years vs 76 years, p<0.001), less likely to have lower respiratory tract symptoms (46.4% vs 66.7%, p=0.007), but more likely to be obese (5.8% vs 0%, p=0.018), pregnant (7.2% vs 0.9%, p=0.027) or have no underlying predisposing factors (24.6% vs 5.1%, p<0.001). Patients with pandemic A (H1N1) were more likely to receive oseltamivir (91.3% vs 40.2%, p<0.001), but less likely to receive antibiotics (75.4% vs 90.6%, p=0.005). Respiratory failure was the reason for intensive care unit admission for all four patients with pandemic A (H1N1), but only for one of three patients with seasonal influenza. There were no statistical significant differences in the rate of intensive care unit admission or death.

Conclusions In addition to age, several clinical parameters were different between pandemic A (H1N1) and seasonal influenza. However, since both seasonal and pandemic influenza can lead to significant morbidity and mortality, the impact of pre-existing seasonal influenza should not be underestimated during the pandemic period.

  • Pandemic influenza
  • seasonal influenza
  • H1N1
  • young
  • obesity
  • pregnancy
  • microbiology
  • infectious diseases and infestations
  • general medicine (see internal medicine)
  • epidemiology
  • respiratory infections

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  • Funding Providence Foundation Limited in memory of the late Dr Lui Hac Minh, the University Grant Council, and the Research Fund for the Control of Infectious Diseases (RFCID) of the Food and Health Bureau of the Hong Kong SAR Government.

  • Competing interests None declared.

  • Ethics approval This study was conducted with the approval of the institutional review board of the Hospital Authority of Hong Kong.

  • Provenance and peer review Not commissioned; externally peer reviewed.