Letters
Respiratory disease
Volume 192 - Issue 6

Comparison of adult patients hospitalised with pandemic (H1N1) 2009 influenza and seasonal influenza during the "PROTECT" phase of the pandemic response

Authors:  Craig B Dalton, Michelle A Cretikos, David N Durrheim, Ian M Seppelt, William D Rawlinson and Dominic E Dwyer

Med J Aust 2010; 192 (6): 356-358. || doi: 10.5694/j.1326-5377.2010.tb03547.x
Published online: 15 March 2010
Download PDF (104 KB)
Share: Share on Blue Sky Share on Facebook Share on LinkedIn Share by email

To the Editor: Chang and colleagues suggest that the clinical course and outcomes of patients infected with pandemic (H1N1) 2009 influenza virus are comparable to those with seasonal influenza infection, and that increased hospital and intensive care unit (ICU) admissions with influenza reflected a higher incidence of disease in the community rather than enhanced virulence of the pandemic influenza virus.1 However, this conclusion was based on an analysis of data from a single hospital, which did not examine whether the community incidence of influenza was increased or whether infection with pandemic (H1N1) 2009 influenza increased the risk of admission to a hospital or ICU compared to infection with seasonal influenza.

Seasonal influenza co-circulated with the pandemic influenza strain during the recent influenza epidemic in New South Wales.2 We used the results of influenza tests (excluding rapid antigen tests) performed at eight major NSW public laboratory services to estimate the risk of admission to an ICU with pandemic (H1N1) 2009 influenza compared with seasonal influenza A infection. These public laboratories confirmed 95% of people admitted to hospital with pandemic (H1N1) 2009 influenza, and 84% of all laboratory-confirmed pandemic influenza cases in NSW.

For the weeks ending 5 June to 2 October 2009, 38 060 specimens were tested for respiratory viruses, of which 7602 were positive on polymerase chain reaction for influenza A. Of these, 4172 (55%) were identified as pandemic (H1N1) 2009 influenza. During the same period, we collected data on all patients admitted to an ICU with influenza A infection.3 In total, 274 patients with influenza A were admitted to NSW ICUs — 229 with pandemic (H1N1) 2009 influenza, 38 with seasonal influenza A and seven with unsubtyped influenza A infection.

Excluding the seven unsubtyped influenza A cases, we calculated a relative risk of 4.9 (95% CI, 3.5–7.0) for admission to ICU with pandemic (H1N1) 2009 influenza compared with seasonal influenza A infection. A sensitivity analysis assigning the unsubtyped influenza A cases admitted to ICUs to either pandemic (H1N1) 2009 influenza or seasonal influenza A produced no significant change in our findings, with a relative risk of admission to ICU with pandemic influenza compared with seasonal influenza A infection of 5.1 (95% CI, 3.6–7.1) and 4.2 (95% CI, 3.1–5.7), respectively.

Although we attempted to collect data on all patients admitted to an ICU with influenza A infection, repeated testing for pandemic (H1N1) 2009 influenza in ICU patients may have resulted in identification of pandemic (H1N1) 2009 influenza in a greater proportion of ICU patients than community patients. However, our results suggest there was a significantly increased risk of admission to an ICU with pandemic (H1N1) 2009 influenza infection compared with other seasonal influenza A strains circulating in NSW. Given the potential for a subsequent pandemic wave, this provides support for maximising rates of community vaccination with a pandemic-specific vaccine.

Authors

  • Craig B Dalton1
  • Michelle A Cretikos2
  • David N Durrheim3
  • Ian M Seppelt1
  • William D Rawlinson4
  • Dominic E Dwyer5
  • 1. Hunter New England Population Health, Hunter New England Area Health Service, Newcastle, NSW.
  • 2. Centre for Epidemiology and Research, New South Wales Department of Health, Sydney, NSW.
  • 3. Department of Intensive Care Medicine, Nepean Hospital, Sydney, NSW.
  • 4. Virology Division, SEALS Microbiology, Sydney, NSW.
  • 5. Centre for Infectious Diseases and Microbiology Laboratory Services, Institute of Clinical Pathology and Medical Research, Department of Virology, Westmead Hospital, Sydney, NSW.

Correspondence: craig.dalton@hnehealth.nsw.gov.au

Competing interests

David Durrheim receives money for direct expenses and travel as a member of the World Health Organization Strategic Advisory Group of Experts (SAGE) on Immunization to the Director General.

Acknowledgements

We acknowledge the work of the eight NSW public hospital laboratories that contributed data, as well as the work of the NSW public health network and the NSW intensive care community.

References

View this article on Wiley Online Library

More like this

Respiratory disease Research 18 August 2026 Open Access

Uneven Ground: Survival Differences Among Victorian Lung Cancer Patients by Location of Residence (2011–2023): A Retrospective Cohort Study

Evangeline Samuel, Eldho Paul, Mike Lloyd, Sanuki Tissera, Craig Underhill, Sagun Parakh, Phillip Parente, Inger Olesen, Javier Torres, Katharine See, Gavin M. Wright, David Langton, Thomas John, Matthew Conron, James Bartlett, Nicola Atkin, Nikolajs Zeps, Susan V. Harden, Wasek Faisal, John R. Zalcberg, Rob G. Stirling

Respiratory disease Research 28 June 2026 Free

Treatment and Survival Outcomes for Indigenous and Non-Indigenous Australians Within the Victorian Lung Cancer Registry: A Retrospective Cross-Sectional Cohort Study

Melanie Wong, Mike Lloyd, Jessie Zeng, Sanuki Tissera, Kalinda E. Griffiths, Justine Clark, Jonathan Gillies, Lisa Briggs, Jacqueline Lesage, Tom Wood, Craig Underhill, Sagun Parakh, Louis B. Irving, Wasek Faisal, Rob Blum, Gary E. Richardson, Phillip Parente, Michelle Caldecott, Inger Olesen, Javier Torres, Evangeline Samuel, Christopher Lyne, Katharine See, David Langton, Thomas John, Gavin Wright, Matthew Conron, James Bartlett, Golsa Adabi, Maggie Moore, Susan Harden, Zoe K. McQuilten, John R. Zalcberg, Rob Stirling

View more