In NSW, when the entire influenza period is considered for each year and the age distribution of influenza-related illness is ignored, the population rate of influenza-associated respiratory admissions to intensive care was greater during seasonal influenza in 2007 than in pandemic influenza in 2009, despite the well-documented severe pandemic influenza-related illness in 2009. The perception of 2009 as a more severe influenza year than 2007 could be due to a number of factors, including the rapid progression of the epidemic of influenza A(H1N1)pdm09 in Australia, the intense media attention and uncharacteristic reporting of counts of confirmed cases and deaths, and the apparent increase in incidence of clearly influenza-related severe acute respiratory distress in younger adults compared with seasonal influenza epidemics. The rapid increase of the 2009 epidemic was followed by an equally rapid decline, thus tempering the final impact. In seasonal influenza years such as 2007, in which older persons were at greater risk of poor outcomes of influenza infection, the role of influenza may be less clear. A large proportion of patients hospitalised with seasonal influenza have symptoms uncharacteristic of a classic influenza syndrome
In 2009, there is some evidence that patients who required intensive care due to influenza-related illness were more severely ill, with a large number of patients diagnosed with ARDS or who received ECMO. Further, while younger patients had a longer median stay in intensive care in 2009 than in other years, these figures should be interpreted with caution as only a small proportion of admissions would be truly due to influenza, and variation in other, non-influenza causes may have explained the differences.
In Australia, 2007 was considered a moderate to severe influenza season, which was attributed to antigenic drift in both the H1N1 and H3N2 influenza strains which may have compromised the effectiveness of that year's Southern Hemisphere influenza vaccine
. Even so, the 2009 period was highly unusual in that increases in influenza-associated intensive care admissions were observed only in people under 65 years. Older people were relatively protected, compared to normal seasonal influenza epidemic periods. Studies both in Australia and internationally have found that older populations tended to have a high prevalence of pre-existing cross-reacting influenza A(H1N1)pdm09 antibodies, while few younger people had such protection
[23–25]. Hence, the all ages excess rate in 2009 was small when compared to 2007, as the increased rate in younger people was offset by the unusually low rate in older people. This was not the case in 2007, where rates were higher than expected in all age groups. A study from Denmark examining all influenza-associated hospital admissions also did not find an increase during the first epidemic of influenza A(H1N1)pdm09 virus compared with seasonal influenza, except in people younger than 65 years
. Our findings are consistent with those of a study in a US hospital that found that patients at greatest risk of complications from seasonal influenza tend to be older, with a mean age of 75 years
. The 2010 epidemic, the first season following the arrival of pandemic influenza continued to be dominated by the pandemic strain. This season was characterised by low influenza activity and very low influenza-related ICU use.
For 2009, we estimated the rate of influenza-related respiratory ICU admission among Aboriginal people to be more than 6 times the highest rate in any single age group of the population overall. In 2009, it was recognised that Indigenous people around the world experienced a greater risk of infection with the pandemic influenza virus and more severe outcomes
[28–30] although a Canadian study found no increased risk of intensive care admission or death among the Canadian Aboriginal populations
. A more recent study of Australian Aboriginal and Torres Strait Islander people found that a younger age distribution and a higher prevalence of underlying chronic conditions are a possible explanation for the apparent increased risk of influenza-related hospitalisation
. Improvements in the recording of Aboriginal status in hospitalisation data in NSW could affect the comparability of rates over time.
The younger risk profile of 2009 pandemic influenza may also at least partly explain the higher rate of influenza-related ICU admission among pregnant women compared with the overall population. However, in all years the risk of admission was greater in pregnant women than in the general population, indicating that seasonal influenza can also pose a strong risk to pregnant women.
The use of administrative data in this study has both strengths and limitations. The main strength of this study is that it is population based which allows complete capture of all intensive care admissions in NSW residents. However, there is certainly some difficulty in accurately and consistently identifying admissions associated with influenza infection over time. There are many non-influenza causes of respiratory illness, both infectious and non-infectious. Other viruses, such as respiratory syncytial virus, may have also been circulating during the influenza periods. We controlled for these non-influenza factors using the Serfling method. However, a limitation of the Serfling method is that it generates a rigid expectation of non-influenza seasonal illness. While the sinusoidal model follows the background seasonal pattern, some variation in non-influenza causes of illness from year-to-year could lead to some over- or underestimation in some years. Other time series methods for estimating influenza-attributable health outcomes are available, such as a combined Serfling-Poisson model that incorporates virology data, but these have been shown to produce broadly similar results
[33, 34]. The lack of a gold standard for assessing the accuracy of these estimates remains a barrier for objectively comparing methods.
Management of ECMO services was coincidentally altered in NSW in 2009 prior to the emergence of the pandemic influenza strain. The aim was to increase the mobility of ECMO services to allow wider geographic application. This may also have contributed to the apparent increase in use of ECMO associated with the 2009 epidemic. In addition, this increased use may reflect the younger age distribution in 2009, given that clinicians may be more likely to use extreme measures on younger patients.
Since there is no generally accepted definition of an influenza season, we used a threshold of 3% of respiratory samples testing positive for influenza to define the seasons. While different thresholds can give slightly different results
, the 2009 season as defined by our method is consistent with other estimates
This study highlights the challenge of identifying influenza-related illness. Had we only included ICU admissions where influenza was recorded as a diagnosis, we would have concluded that there was a much greater use of influenza-related intensive care during 2009 than 2007. Prior to 2009 specific testing for influenza in ICUs was uncommon, and during 2009 we believe that pandemic awareness amongst clinicians promoted testing for influenza, and increased the likelihood that influenza was recorded as part of the diagnosis, as evidenced by Figure
1. As well, a shift between 2007 and 2009 from the use of immunofluorescence for the detection of influenza to the more sensitive nucleic acid testing may have contributed to the increased rate of influenza diagnosis in 2009.
Including all respiratory admissions is a more sensitive method of estimating the impact of influenza. The influence of influenza during 2007 may have been more evident in the broader respiratory diagnosis group due to the older age of those most at risk of influenza complications in 2007. Older people have more complex and chronic illnesses which could mask the recognition of influenza's role in the illness and lead to more frequent diagnoses of alternative conditions such as pneumonia and respiratory failure/end-stage respiratory disease that could be secondary respiratory complications of primary influenza infection