Pandemic influenza H1N1/09 emerged in April 2009 and spread widely in Australia and New Zealand. Although an unprecedented number of cases required intensive care, comparative community-based studies with seasonal influenza strains have not shown any significant differences in clinical symptoms or severity.
The authors performed active surveillance on confirmed influenza-related admissions and compared the clinical profile of patients with pandemic H1N1/09 influenza and patients with seasonal influenza at eight hospitals in Australia and one hospital in New Zealand.
During the 1 July and 30 November 2009, 560 patients with confirmed influenza were admitted, of which 478 had H1N1/09, and 82 had other seasonal strains. Patients with H1N1/09 influenza were younger, were more likely to have fever and were more likely to be pregnant but less likely to have chronic obstructive pulmonary disease and ischaemic heart disease than patients with seasonal strains. Other clinical features and comorbidities were reported in similar proportions. Admission to intensive care was required in 22% of patients with H1N1/09 influenza and 12% in patients with other strains. Hospital mortality was 5% in patients with H1N1 influenza.
The clinical features of H1N1/09 influenza and seasonal strains were similar in hospitalised patients. A higher proportion of patients had comorbidities than had been reported in community-based studies. Although the overall mortality was similar, the authors found evidence that H1N1/09 caused severe disease in a higher proportion of hospitalised patients.
We performed an observational study of patients with H1N1/09 and seasonal strains of influenza in 2009, based on active surveillance at nine sentinel hospitals.
We explored differences between patients with H1N1/09 influenza infection and those with seasonal influenza infections.
This study found that the clinical features of H1N1/09 influenza were similar in hospitalised patients, similar to previous community-based studies.
The finding that H1N1/09 influenza was associated with more severe disease reconciles apparently contradictory data suggesting no differences in community studies, but unprecedented use of critical care services.
Strengths and limitations of this study
This surveillance system was rapidly established, and initial data collection was retrospective from the medical record where symptoms were not always well documented. Despite high levels of awareness in medical staff, clinical testing criteria were operating during the period of the study and were likely to bias the proportion of patients reporting fever and respiratory symptoms. Nucleic-acid detection using PCR is regarded as the gold standard for diagnosis, but our experience with discordant results on repeated testing suggested that it may not be completely sensitive. This study does not encompass the full duration of the epidemic which was waning in several states (notably Victoria and New South Wales) at the commencement of the study period. Although several hospitals provided maternity and paediatric services, these patient groups are likely to be under-represented in this series. The population served by the sentinel hospitals is not known, and thus we were not able to establish a disease incidence rate.
This large study captured all admissions with influenza at multiple hospitals across Australia and New Zealand. All cases were confirmed by nucleic acid detection with clinical details collected by research staff.