Hospitalisation with confirmed influenza in a sentinel surveillance system in 2010 and 2011

To the Editor: Influenza is a common respiratory infection that is infrequently associated with severe disease requiring hospitalisation. We have conducted hospital-based surveillance in the Influenza Complications Alert Network (FluCAN) since 20091 and report on our findings in 2010 and 2011.

In 2010, 15 hospitals based in capital cities or large regional centres were involved, as previously described.1,2 In 2011, eight hospitals in Victoria, South Australia, Western Australia and the Australian Capital Territory reported details of patients admitted with influenza confirmed by polymerase chain reaction (PCR) testing. Data on patients aged less than 18 years were only collected during 2010. During the periods March to November 2010 and May to November 2011, 598 patients were admitted to the sentinel hospitals with confirmed influenza. Of these patients, 52% were female, and their median age was 44 years (interquartile range [IQR], 27–62 years). Of the 333 patients whose vaccine status was ascertained, 109 patients (32.7%) reported having been vaccinated with the 2011 seasonal vaccine in 2011, or the monovalent H1N1 2009 vaccine and/or the 2010 seasonal vaccine in 2010. Of all patients, 72.9% had a chronic comorbidity (Box). Overall, 84 patients (14%) were admitted to intensive care and 18 patients (3%) died in hospital. The median length of stay was 4 days (IQR, 2–8 days; mean, 8.4 days). In 2010, 17 of the 56 children aged under 18 years (30%) were admitted to intensive care, but there were no deaths in this age group.

We have previously found that sentinel surveillance reflects population-based surveillance systems for influenza.1 We caution about making comparisons between patient characteristics reported in the 2010 and 2011 seasons, as the hospitals involved in each year were different. However, the comorbid conditions reported in patients hospitalised in this study are broadly consistent with those reported in northern hemisphere surveillance systems.3

Based on the ratio of cases to hospital beds, we estimate that there were about 1800 cases of confirmed influenza in 2010 and 3200 cases in 2011, nationally. This is likely to be an underestimate of the true burden of severe influenza, as not all patients hospitalised with influenza are tested, and patients may present with postinfluenza sequelae such as secondary bacterial pneumonia.