The impact of influenza A(H1N1)pdm09 compared with seasonal influenza on intensive care admissions in New South Wales, Australia, 2007 to 2010: a time series analysis

Abstract

BackgroundIn Australia, the 2009 epidemic of influenza A(H1N1)pdm09 resulted in increased admissions to intensive care. The annual contribution of influenza to use of intensive care is difficult to estimate, as many people with influenza present without a classic influenza syndrome and laboratory testing may not be performed. We used a population-based approach to estimate and compare the impact of recent epidemics of seasonal and pandemic influenza.MethodsFor 2007 to 2010, time series describing health outcomes in various population groups were prepared from a database of all intensive care unit (ICU) admissions in the state of New South Wales, Australia. The Serfling approach, a time series method, was used to estimate seasonal patterns in health outcomes in the absence of influenza epidemics. The contribution of influenza was estimated by subtracting expected seasonal use from observed use during each epidemic period.ResultsThe estimated excess rate of influenza-associated respiratory ICU admissions per 100,000 inhabitants was more than three times higher in 2007 (2.6/100,000, 95% CI 2.0 to 3.1) than the pandemic year, 2009 (0.76/100,000, 95% CI 0.04 to 1.48). In 2009, the highest excess respiratory ICU admission rate was in 17 to 64 year olds (2.9/100,000, 95% CI 2.2 to 3.6), while in 2007, the highest excess rate was in those aged 65 years or older (9.5/100,000, 95% CI 6.2 to 12.8). In 2009, the excess rate was 17/100,000 (95% CI 14 to 20) in Aboriginal people and 14/100,000 (95% CI 13 to 16) in pregnant women.ConclusionWhile influenza was diagnosed more frequently and peak use of intensive care was higher during the epidemic of pandemic influenza in 2009, overall excess admissions to intensive care for respiratory illness was much greater during the influenza season in 2007. Thus, the impact of seasonal influenza on intensive care use may have previously been under-recognised. In 2009, high ICU use among young to middle aged adults was offset by relatively low use among older adults, and Aboriginal people and pregnant women were substantially over-represented in ICUs. Greater emphasis on prevention of serious illness in Aboriginal people and pregnant women should be a priority in pandemic planning.

HLA targeting efficiency correlates with human T-cell response magnitude and with mortality from influenza A infection

“…S2A). Our analysis is supported by several reports following the pH1N1 outbreaks in the spring of 2009 in North America and in Australia and New Zealand (2)(3)(4)(5)(6)(7)(8)(9)(10)(11).…”

“…This also provides a potential explanation for the increased pH1N1 disease severity and morbidity in indigenous populations worldwide (2)(3)(4)(5)(6)(7)(8)(9)(10), populations that are all enriched for A*24 alleles despite their widespread geographic divergence.…”

“…We then conducted a population-based analysis and found that the carriage frequencies of the alleles with the lowest targeting efficiencies, HLA A*24, were associated with pH1N1 mortality. Interestingly, these alleles are common in certain indigenous populations in which increased pH1N1 morbidity has been reported (2)(3)(4)(5)(6)(7)(8)(9)(10). These data suggest that HLA alleles may be useful markers for assessing the risk of potential pandemics.…”

“…The pH1N1 virus was a triple-reassortant virus that had not been reported previously in animals or humans (1), and several epidemiologic characteristics differed substantially between this and other circulating H1N1 viruses. pH1N1-associated hospitalization rates and mortality among the elderly population were lower than those for seasonal influenza and higher for children and young adults (1); they also were higher for Hispanic, African American, Pacific Islander and American Indian/Alaska Native populations than for those of European/Caucasian descent (2)(3)(4)(5)(6)(7)(8)(9)(10). Early analyses found that vaccination with seasonal influenza vaccines was unlikely to provide protection from pH1N1 because of very low levels of cross-reactive antibody responses (11).…”

Mortality Attributable to Seasonal and Pandemic Influenza, Australia, 2003 to 2009, Using a Novel Time Series Smoothing Approach

“…In addition, younger people are more likely to have received intensive care if critically ill with pandemic infuenza, than elderly persons who are more likely to suffer severe outcomes during severe seasonal influenza epidemics. In Australia, use of critical care for respiratory illness in younger adults was clearly greater during the epidemic of the pandemic influenza virus in 2009 than during prior seasonal epidemics [40]. Availability of critical care technologies such as extracorporeal membrane oxygenation (ECMO) is also thought to have had an impact on survival [41].…”