2009 pandemic H1N1 influenza: Risk factors for severe and fatal manifestations

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Introduction: Most of the confirmed 2009 H1N1 pandemic cases showed mild influenza like illness similar to seasonal influenza. However, there were also severe and fatal cases. Knowledge of the risk factors associated with severe and fatal cases is important. This study assessed the risk factors for severe and fatal manifestations. Materials and Methods: Search of Medline/PubMed (search period unrestricted) using keywords 'Pandemic', 'H1N1 influenza', 'risk factor', 'severe and/or fatal' was performed. Results: The incidence of severe and fatal cases ranged from 20 to 100% and from 0.7 to 39% respectively. Studies on age as a factor showed mixed results but generally showed the younger age group to be at higher risk, different to that of seasonal influenza. Various co-morbid conditions such as pregnancy, obesity, diabetes mellitus, chronic pulmonary and cardiac disease, being immune suppressed, nutritional status, and delay in treatment have been shown to be important risk factors for severe disease. One study showed Streptococcus pneumoniae and Human respiratory syncytial virus A co-infection to have a role in the severity of the disease. Indirect racial study suggested that genetic factors are important. Other factors that may be important and required further research such as associated preexisting conditions such as arterial hypertension, active tuberculosis and neurological disease, in addition to over-responsive innate immune response, viral dose, alcohol consumption, and environmental factors. Conclusion: In the 2009 H1N1 influenza pandemic, risk factors for severe and fatal cases include preexisting medical conditions, obesity, underweight, pregnancy, and delay in antiviral treatment. Respiratory co-infection may be important.

Original languageEnglish
Pages (from-to)139-147
Number of pages9
JournalBrunei International Medical Journal
Issue number3
Publication statusPublished - Jun 2011


  • H1N1 subtype
  • Hospitalisation
  • Influenza
  • Influenza A virus
  • Mortality
  • Pandemics


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